Bluetooth thread may ask encoder to reduce bitrate if writing is not keeping up with inputs or writing to bluetooth socket takes too much time.
SBC codec decrements bitpool value by fixed amount each time it is asked to reduce output bitrate. This results in reduced audio quality with SBC codec.
Assuming conditions leading to reduced bitrate are intermittent, allow periodic attempts to increase encoder bitrate, by default at most twice per second.
For SBC codec increase output bitrate by adding 1 to bitpool value each time encoder bitrate needs to be increased, so that SBC quality can be restored. While at it, remove bitpool decrement limit to use connection agreed value instead as we will be able to restore quality later.
This change implements SBC part on top of common changes in !481 (merged)