AudioManager: listener for changes to preferred device for strategy

Add a listener for being notified when the preferred audio device
for an audio strategy changes.

Bug: 148566862
Bug: 144440677
Test: atest AudioServiceHostTest#testPreferredDeviceRouting
Test: atest AudioServiceHostTest#testDevicesForAttributes
Change-Id: Iff47d6bc7f4bd18c3a8fe48557acf803a4059630