State transition for PbapClientStateMachine during Disconnect Timeout

The PbapClient state machine did not transition from the
Disconnecting state to the Disconnected state in the case of a timeout. This
has been resolved in this CL which also includes tests to verify successful state transitions.

Tag: #stability
Bug: 211515554
Test: atest PbapClientStateMachineTest
Change-Id: I46f68aaa71b4d2713eaea21722d26193c33aeae5