Let mediaserver use CleanupMode::CLEAN_ALL mode for RenderEngine

This mode for cleanupPostRender:
* Destroys the underlying image cache, so the expicit unbind() call
isn't needed.
* Rebinds texture memory to a placeholder buffer, so the driver won't
keep a buffer around.

Bug: 162202251
Test: Launch custom apk with thumbnail generation
Test: adb shell dumpsys meminfo mediaserver
Change-Id: Idcb008600aab81a46eef4405ad9d52382e5dd810
Merged-In: Idcb008600aab81a46eef4405ad9d52382e5dd810