Min API 19 -> 21

Fix #1579.

mobile/src/main/java/com/github/shadowsocks/bg/Dns.kt

@@ -20,49 +20,19 @@
 
 package com.github.shadowsocks.bg
 
-import android.content.Context
-import android.net.ConnectivityManager
-import android.net.LinkProperties
-import android.os.Build
 import android.util.Log
 import com.github.shadowsocks.App.Companion.app
 import com.github.shadowsocks.BuildConfig
-import okhttp3.Dns as Okdns
 import org.xbill.DNS.*
 import java.net.Inet6Address
 import java.net.InetAddress
 import java.net.NetworkInterface
 import java.net.UnknownHostException
-import java.util.*
+import okhttp3.Dns as Okdns
 
 object Dns {
     private const val TAG = "Dns"
 
-    @Throws(Exception::class)
-    fun getDnsResolver(context: Context): String {
-        val dnsResolvers = getDnsResolvers(context)
-        if (dnsResolvers.isEmpty()) throw Exception("Couldn't find an active DNS resolver")
-        val dnsResolver = dnsResolvers.iterator().next().toString()
-        if (dnsResolver.startsWith("/")) return dnsResolver.substring(1)
-        return dnsResolver
-    }
-
-    @Throws(Exception::class)
-    private fun getDnsResolvers(context: Context): Collection<InetAddress> {
-        val addresses = ArrayList<InetAddress>()
-        val connectivityManager = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
-        val classLinkProperties = Class.forName("android.net.LinkProperties")
-        val getActiveLinkPropertiesMethod = ConnectivityManager::class.java.getMethod("getActiveLinkProperties")
-        val linkProperties = getActiveLinkPropertiesMethod.invoke(connectivityManager)
-        if (linkProperties != null) {
-            if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
-                val dnses = classLinkProperties.getMethod("getDnses").invoke(linkProperties) as Collection<*>
-                dnses.mapTo(addresses) { it as InetAddress }
-            } else addresses.addAll((linkProperties as LinkProperties).dnsServers)
-        }
-        return addresses
-    }
-
     private val hasIPv6Support get() = try {
         val result = NetworkInterface.getNetworkInterfaces().asSequence().flatMap { it.inetAddresses.asSequence() }
                 .count { it is Inet6Address && !it.isLoopbackAddress && !it.isLinkLocalAddress } > 0

mobile/src/main/java/com/github/shadowsocks/bg/ServiceNotification.kt

@@ -40,8 +40,7 @@ import java.util.*
 
 /**
  * Android < 8 VPN:     always invisible because of VPN notification/icon
- * Android 4.x other:   always visible
- * Android 5-7 other:   only invisible in (possibly unsecure) lockscreen
+ * Android < 8 other:   only invisible in (possibly unsecure) lockscreen
  * Android 8+:          always visible due to system limitations
  *                      (user can choose to hide the notification in secure lockscreen or anywhere)
  */
@@ -85,24 +84,22 @@ class ServiceNotification(private val service: BaseService.Interface, profileNam
         if (Build.VERSION.SDK_INT < 24) builder.addAction(R.drawable.ic_navigation_close,
                 service.getString(R.string.stop), PendingIntent.getBroadcast(service, 0, Intent(Action.CLOSE), 0))
         val power = service.getSystemService(Context.POWER_SERVICE) as PowerManager
-        update(if (if (Build.VERSION.SDK_INT >= 20) power.isInteractive else @Suppress("DEPRECATION") power.isScreenOn)
-            Intent.ACTION_SCREEN_ON else Intent.ACTION_SCREEN_OFF, true)
+        update(if (power.isInteractive) Intent.ACTION_SCREEN_ON else Intent.ACTION_SCREEN_OFF, true)
         val screenFilter = IntentFilter()
         screenFilter.addAction(Intent.ACTION_SCREEN_ON)
         screenFilter.addAction(Intent.ACTION_SCREEN_OFF)
-        if (visible && Build.VERSION.SDK_INT in 21 until 26) screenFilter.addAction(Intent.ACTION_USER_PRESENT)
+        if (visible && Build.VERSION.SDK_INT < 26) screenFilter.addAction(Intent.ACTION_USER_PRESENT)
         service.registerReceiver(lockReceiver, screenFilter)
     }
 
     private fun update(action: String, forceShow: Boolean = false) {
         if (forceShow || service.data.state == BaseService.CONNECTED) when (action) {
             Intent.ACTION_SCREEN_OFF -> {
-                setVisible(visible && Build.VERSION.SDK_INT < 21, forceShow)
+                setVisible(false, forceShow)
                 unregisterCallback()    // unregister callback to save battery
             }
             Intent.ACTION_SCREEN_ON -> {
-                setVisible(visible && (Build.VERSION.SDK_INT < 21 || !keyGuard.inKeyguardRestrictedInputMode()),
-                        forceShow)
+                setVisible(visible && !keyGuard.inKeyguardRestrictedInputMode(), forceShow)
                 service.data.binder.registerCallback(callback)
                 service.data.binder.startListeningForBandwidth(callback)
                 callbackRegistered = true

mobile/src/main/java/com/github/shadowsocks/bg/VpnService.kt

@@ -24,7 +24,6 @@ import android.app.Service
 import android.content.Intent
 import android.content.pm.PackageManager
 import android.net.LocalSocket
-import android.os.Build
 import android.os.IBinder
 import android.os.ParcelFileDescriptor
 import android.util.Log
@@ -140,7 +139,7 @@ class VpnService : BaseVpnService(), LocalDnsService.Interface {
             builder.addRoute("::", 0)
         }
 
-        if (Build.VERSION.SDK_INT >= 21 && profile.proxyApps) {
+        if (profile.proxyApps) {
             val me = packageName
             profile.individual.split('\n')
                     .filter { it != me }