-Added toggle to allow main video player to switch to popup player when onstop is called.

-Fixed player state not recovering when player is stopped during multiwindow mode.
-Updated gradle to 3.1.2.

app/src/main/java/org/schabi/newpipe/player/MainVideoPlayer.java

@@ -208,10 +208,17 @@ public final class MainVideoPlayer extends AppCompatActivity
     protected void onStop() {
         if (DEBUG) Log.d(TAG, "onStop() called");
         super.onStop();
-        playerImpl.destroy();
-
         PlayerHelper.setScreenBrightness(getApplicationContext(),
                 getWindow().getAttributes().screenBrightness);
+
+        isInMultiWindow = false;
+
+        if (playerImpl == null) return;
+        if (PlayerHelper.isMinimizeOnExitEnabled(this)) {
+            playerImpl.onFullScreenButtonClicked();
+        } else {
+            playerImpl.destroy();
+        }
     }
 
     /*//////////////////////////////////////////////////////////////////////////

app/src/main/java/org/schabi/newpipe/player/helper/PlayerHelper.java

@@ -173,6 +173,10 @@ public class PlayerHelper {
         return isAutoQueueEnabled(context, false);
     }
 
+    public static boolean isMinimizeOnExitEnabled(@NonNull final Context context) {
+        return isMinimizeOnExitEnabled(context, false);
+    }
+
     @NonNull
     public static SeekParameters getSeekParameters(@NonNull final Context context) {
         return isUsingInexactSeek(context, false) ?
@@ -249,7 +253,6 @@ public class PlayerHelper {
      * System font scaling:
      * Very small - 0.25f, Small - 0.5f, Normal - 1.0f, Large - 1.5f, Very Large - 2.0f
      * */
-    @NonNull
     public static float getCaptionScale(@NonNull final Context context) {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) return 1f;
 
@@ -322,4 +325,8 @@ public class PlayerHelper {
             return sp.getFloat(context.getString(R.string.screen_brightness_key), screenBrightness);
         }
     }
+
+    private static boolean isMinimizeOnExitEnabled(@NonNull final Context context, final boolean b) {
+        return getPreferences(context).getBoolean(context.getString(R.string.minimize_on_exit_key), b);
+    }
 }