PBAPClient State Machine

Refactor PBAP Client code to function as a state machine.  By
consolidating threads and state tracking there is less duplication and
less opportunity for conflicting combinations of state.

bug: 28249138
Change-Id: I20de8041b83024a03bfbb60102c44b4e26579553
(cherry picked from commit 427695a7992c8fc3fb503794bbaff1f705451133)