sbc: detect when bitpool has changed

sbc: detect when bitpool has changed

A2DP spec allow bitpool changes midstream which is why sbc configuration
has a range of values for bitpool that the encoder can use and decoder
must support.

Bitpool changes do not affect the state of encoder/decoder so they don't
need to be reinitialize when this happens, so the impact is fairly small,
what it does change is the frame length so encoders may change the
bitpool to use the link more efficiently.