Have destroy call freePrefetchedLayers

 Bug: 17208461

 There's a potential race condition between HardwareRenderer.destroy()
 being called (which calls destroyCanvasAndSurface()) and the renderer
 being finalized (which is what calls freePrefetchedLayers), during which
 time it's possible we get a TRIM_MEMORY_COMPLETE and destroy the EGL
 context.

 Fix this race condition by moving stopDrawing() and freePrefetchedLayers()
 into destroyCanvasAndSurface() where they should have been in the first
 place.

 Also, if we hit the assertion failure, dump the current state of
 Caches to try and provide more context for the failure.

Change-Id: Ife0ba3562041e8b08e87e3e13640472b3004eed6