1. When re-allocating for a track in DEFICIENT state, try to use
available headroom to accommodate change before trying to steal
bits from other tracks.
2. If the changing track gives back bits (because of muting or
moving to a lower layer subscription), use the returned bits
to try and boost deficient track(s).
* Plug a couple of holes in stream transitions.
1. Missed negative sign meant stealing bits from other tracks was not
working.
2. When a track change (mute, unmute, subscription change) cannot be
allocated, explicitly pause so that stream state update happens.
Refactor stream state update a bit to make it a bit cleaner.
* correct comment
