LiveWallpapers with non-continuous rendering are missing some offsetsChanged notifications

If an application is used as a LiveWallpaper and set to non-continuous rendering, *offsetsChanged* notifications are not dispatched to the render thread in the following situations:

1. **The user presses the *home* button to go to the central screen:** since the *home* button is not part of the live wallpaper, no input event is processed by libgdx and no render is performed. 
2. **The user swipes to go to a sibling screen:** here, input events are generated while the user keeps his/her finger on the screen, producing normal rendering. However, if the user stops touching the screen in the middle of a screen transition, the screen is automatically advanced to the next one or automatically returned to the original one. In both cases, *offsetsChanged* events are generated, but since there is no input event, there is also no rendering.

This patch solves the problem by ensuring the *Gdx.graphics.requestRendering()* is called right after every call to *notifyOffsetsChanged()* in the AndroidLiveWallpaperService.