audio: improve audio routing callbacks

Do not force audio device changed callback when the client
(AudioTrack or AudioRecord) registers to AudioFlinger but only when
it starts playback or capture. Doing so prevents a spurious callback
happing at registration time where a stale device
(previously selected by AudioFlinger thread) but irrelevant to this
client is indicated. This causes a disconnection of AAudio streams
despite no real device change.

Bug: 128630993
Test: CTS: android.nativemedia.aaudio.AAudioTests, android.media.cts.RoutingTest
      CTS Verifier: Audio Input/Output Routing Notifications Test

Change-Id: Ia7f1d11490989b0287c97479466c1c07a223aab3