Add more tests, fix lint errors and add check to travis

mobile/src/main/java/com/github/shadowsocks/AppManager.kt

@@ -180,7 +180,9 @@ class AppManager : AppCompatActivity(), Toolbar.OnMenuItemClickListener {
         setContentView(R.layout.layout_apps)
         toolbar = findViewById(R.id.toolbar)
         toolbar.setTitle(R.string.proxied_apps)
-        toolbar.setNavigationIcon(R.drawable.abc_ic_ab_back_material)
+        val arr = obtainStyledAttributes(intArrayOf(R.attr.homeAsUpIndicator))
+        toolbar.navigationIcon = arr.getDrawable(0)
+        arr.recycle()
         toolbar.setNavigationOnClickListener {
             val intent = parentActivityIntent
             if (shouldUpRecreateTask(intent) || isTaskRoot)

mobile/src/main/java/com/github/shadowsocks/ScannerActivity.kt

@@ -54,7 +54,9 @@ class ScannerActivity : AppCompatActivity(), ZXingScannerView.ResultHandler {
         setContentView(R.layout.layout_scanner)
         val toolbar = findViewById<Toolbar>(R.id.toolbar)
         toolbar.title = title
-        toolbar.setNavigationIcon(R.drawable.abc_ic_ab_back_material)
+        val arr = obtainStyledAttributes(intArrayOf(R.attr.homeAsUpIndicator))
+        toolbar.navigationIcon = arr.getDrawable(0)
+        arr.recycle()
         toolbar.setNavigationOnClickListener { navigateUp() }
         scannerView = findViewById(R.id.scanner)
         if (Build.VERSION.SDK_INT >= 25) getSystemService(ShortcutManager::class.java).reportShortcutUsed("scan")

mobile/src/main/java/com/github/shadowsocks/database/KeyValuePair.kt

@@ -53,7 +53,7 @@ class KeyValuePair() {
         get() = if (valueType == TYPE_LONG) ByteBuffer.wrap(value).long else null
     val string: String?
         get() = if (valueType == TYPE_STRING) String(value) else null
-    val stringSet: MutableSet<String>?
+    val stringSet: Set<String>?
         get() = if (valueType == TYPE_STRING_SET) {
             val buffer = ByteBuffer.wrap(value)
             val result = HashSet<String>()
@@ -95,7 +95,7 @@ class KeyValuePair() {
         this.value = value.toByteArray()
         return this
     }
-    fun put(value: MutableSet<String>): KeyValuePair {
+    fun put(value: Set<String>): KeyValuePair {
         valueType = TYPE_STRING_SET
         val stream = ByteArrayOutputStream()
         for (v in value) {