surfaceflinger: simplify HWC buffer cache clean up

When a Layer is no longer connected, we destroy the associated HWC
layers on next call to SurfaceFlinger::rebuildLayerStacks or when
the Layer is destroyed.  There is no need to listen to
onBuffersReleased.  Besides, we need to perform the cleanup from the
main thread as we only talk to HWC process from the main thread.

While at it, move HWComposerBufferCache to its own files.

Bug: 35320590
Test: manual
Change-Id: Ifa32f24076b094c8fa9cda8572b03d5bfb8e0b93