Use SIGKILL to kill old processes

Fix #1455.

Note that in later Android versions, only processes under same uid will be listed and this will be quite efficient.

mobile/src/main/java/com/github/shadowsocks/JniHelper.java

@@ -64,6 +64,7 @@ public class JniHelper {
         }
     }
 
+    public static native int sigkill(int pid);
     private static native int sigterm(Process process);
     private static native Integer getExitValue(Process process);
     private static native Object getExitValueMutex(Process process);