A2DP: Use mutex to only to check and set HAL state

In multi-threaded env A2DP state gets updated to suspended
after checking and before acquiring mutex lock. A2DP State
should be checked inside mutex lock to avoid checking against
inconsistent state.
Moreover this change prevents overwriting of state
from AUDIO_A2DP_STATE_SUSPENDED to AUDIO_A2DP_STATE_STOPPED
which would start the data path when there is data inflow
from audio in suspended state, which is not intended.
This change helps avoiding above wrong actions.

Bug: 22028876
Change-Id: I4fb73c1a6851fa97d539860d5fc0b8022b019bc1