allow for slow FrameMetricsListeners

A slow listener could cause a race in the NotifyHandler
where the single reference to the buffer to send would get
updated when it shouldn't have been.

Switch to a queue of available buffers to prevent this race.

Also, stop setting and clearing the observer reference and instead
incStrong/decStrong to mark temporary strong ownership without
colliding with other owners in flight.

Bug: 27097094
Change-Id: Iee647bfae8b80019b6d8290179eed3973230901f