Release virtual display buffer immediately after HWC set

Previously we only queued a virtual display buffer to the sink when
the next frame was about to be displayed. This may delay the "last"
frame of an animation indefinitely. Now we queue the buffer as soon as
HWC set() returns and gives us the release fence.

Bug: 8384764
Change-Id: I3844a188e0f6ef6ff28f3e11477cfa063a924b1a