graphics: fix potential leaks for IAllocator

Introduce IAllocatorClient to manage resources owned by a client (e.g., SF
or VTS).  This makes sure there is no resource leak when SF or VTS
crashes.

This also fixes two unrelated bugs

 - sizeof(Buffer) != sizeof(void*) on 32-bit impl.
 - layerCount was not set to 1 in tests

Test: builds and boots
Change-Id: I67f5cdd64b97fb3ce1b931099c25f59cc8517f21