BassClient: Send remove source command after stopping synchronization

Source cannot be remove if it's not stopped - in term of synchronization
(PA & BIG). It could be done after BassClient being notified that there
is no synchronization or stopped. Current implementation sends
immediately command one after one (modify and remove) without wait for
notifiaction that delegator is no longer synchronized.

Test: presubmit
Test: atest BluetoothInstrumentationTests
Test: atest BassClientStateMachineTest
Bug: 150670922
Bug: 229965085
Tag: #feature
Sponsor: jpawlowski@
Change-Id: Ib1ecd096a8101a898ca599883da8249b343642b6