Release the bitmap reference and textures when user changes the wallpaper.

We don't need the bitmap after we uploads it to texture so we nullify
the reference to get it can be gced.

We also need to release the graphic memory when user changes the wallpaper
or the graphic memory will increase dramatically.

Bug: 128897294
Test: Change wallpaper, observes android profiler
Change-Id: I207d67741e6a7643a13ba97fccdab355dd09e30c