BASS: Fix flaky tests

Fix addBcastSource_withCachedScanResults by waiting for
SELECT_BCAST_SOURCE operation finished.

Test attCallbackOnConnectionStateChanged_changedToDisconnected
failed because of above test.

Fix testStopSearchingForSources_timeoutForActiveSync by removing
unnecessary validation at the end. That part is tested in stop and
stopSearching tests. Here we are interested only at unsync in desired time.

Remove unnecessary timeouts on mMethodProxy verification, as they are
called directly in the code.

Bug: 338574075
Bug: 333691526
Test: atest BassClientServiceTest BassClientStateMachineTest
Change-Id: I6b2ecf00e57e730300be1f1c2c9c27a3d91a1e67