blast: drop buffer from SF's cache when destroyed

When an app drops its reference to an AHardwareBuffer, the buffer
should be removed from the client and SurfaceFlinger caches.

Ideally, both caches would have wp to the buffer and the buffer
would automatically be removed from the cache.

Unfortunately, GraphicBuffers are refcounted per process. If SurfaceFlinger
just has a wp to the GraphicBuffer, the buffer's destructor will
be called and SurfaceFlinger will lose access to the buffer.
SurfaceFlinger can't just hold onto a sp to the buffer because
then the buffer wouldn't be destoryed when the app drops its reference.

Instead, when the app process drops its last strong reference,
GraphicBuffer will send a callback to the client side cache.
The cache will send a Transaction to SurfaceFlinger to drop its sp
to the buffer.

Bug: 127689853
Test: SurfaceFlinger_test

Change-Id: I2182578ed33d7c731945cb88cd1decb2892266b0