Fix possible memory leak for Android and update gradle

[volzhs]

there is an error message when compile with latest build tool for android.

/godot_dev/platform/android/java/src/com/google/android/vending/expansion/downloader/impl/DownloaderService.java:575: Error: The WIFI_SERVICE must be looked up on the Application context or memory will leak on devices < Android N. Try changing  to .getApplicationContext()  [WifiManagerLeak]
            mWifiManager = (WifiManager) getSystemService(Context.WIFI_SERVICE);
                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   Explanation for issues of type "WifiManagerLeak":
   On versions prior to Android N (24), initializing the WifiManager via
   Context#getSystemService can cause a memory leak if the context is not the
   application context. Change context.getSystemService(...) to
   context.getApplicationContext().getSystemService(...).

1 errors, 0 warnings

this pr will fix the error and update to latest gradle.