Release the media player when the video preference is not visible.

Mediaplayer is not released when the device is paused, and hence it
keep consuming battery.

Fixes: 120945749
Test: adb root && watch -n 0.5 -d "adb shell cat /sys/kernel/debug/msm_vidc/core0/inst*/info\|egrep -i \"height\|width\|instance\|fps\|name\""
      Use above command to check if any decoder instance exists.

Change-Id: Ia0edbba0c2fd3c70753bc36d23d82c0770f95672

tests/robotests/src/com/android/settings/widget/VideoPreferenceTest.java

@@ -92,4 +92,13 @@ public class VideoPreferenceTest {
 
         verify(mMediaPlayer, never()).start();
     }
+
+    @Test
+    public void onViewInvisible_shouldReleaseMediaplayer() {
+        mVideoPreference.onViewVisible(false);
+
+        mVideoPreference.onViewInvisible();
+
+        verify(mMediaPlayer).release();
+    }
 }