Reducing memory usage of wallpaper picker

> Loading preview bitmap only once, instead of loading it twice at BitmapRegionTileSource and BitmapSource
> Maintaing a weak-set of reusable bitmaps and reusing them for decoding bitmaps
> Loading images on a HandlerThread (instead of AsyncTask) and removing any non-started task before submitting a new task
> Loading inbuild images (from resources) on HandlerThread instead of UIThread
> Freeing up unbound GL textures before binding a new texture.

Bug: 18382606
Change-Id: Ic4ca630dd113ded65d2853eb0d291c9e5823637e