Track the last release fence per layer stack

Merging fences prior to calling CompositionEngine#present results in
occassional missed frames in Surfaceflinger. Merging fences here was
originally needed to relieve memory pressure. Instead, we can track
each layer stack's future release fence with a map between a layer
stack and its last fence, as it can be assumed that multiple fences
for the same layer stack can be be dropped.

Fixes: b/330841053
Test: SurfaceFlinger_test
Test: presubmit
Change-Id: I7ca3a226ff77bc31d93fdb4708c3e9089f423803