ActiveDeviceManagerTest: Dispatch tasks before verify

Wait for looper to finish scheduled task on each post call when
there is any verify after it. We can remove timeouts in verification.

Fixed a lot of rare flakiness.

Fixed a2dpAndLeAudioConnectedThenLeAudioDisconnected_fallbackToA2dp
which sometimes ends up before tasks were executed, which caused
wrong behavior on getLeadDevice mock.

Speeds up tests execution more than two times.

Bug: 321202011
Test: atest ActiveDeviceManagerTest --rerun-until-failure 1000
Flag: Exempt, tests changes only
Change-Id: I6eb9f146a6c3408cdafe2b59f69e8aad53ccd3c0