AudioManager: onPlaybackConfigChanged called w/ latest config

When AudioService sends new playback configurations, they are
enqueued into the Handler of the registered callback.
Whenever a new config is received, discard unhandled messages and
enqueue the new one, instead of just queueing: clients only
need the new state.

Bug: 361092797
Flag: EXEMPT bug fix
Test: atest AudioPlaybackConfigurationTest
Change-Id: I54e6cc997dcbb5c1684acc1acd82b1f7bae50eeb