Fix FenceTracker releaseFence

This patch:
* Fixes the release fence when GPU compositing.
* Stores the final release fence in ConsumerBase just
    before releasing the Buffer, which helps ensure
    sync points aren't added unknowningly.
* Makes HWC2 release pending buffers as the first step
    of postCompostion, rather than the last, which should
    allow dequeue to unblock a little earlier and helps
    make sure the previous buffer's release fence has
    been finalized before FenceTracker::addFrame is
    called.
* Fence tracker only sets the release fence once it
    has been finalized so it does not report a release
    fence for a buffer that is still latched.

Test: adb shell /data/nativetest/libgui_test/libgui_test
--gtest_filter=*GetFrameTimestamps*

Change-Id: I27d484bfd48f730bdcea2628f96795c6f4b4df7b