New intent action for speakerphone state change

New intent action ACTION_SPEAKERPHONE_STATE_CHANGED to indicate
  speakerphone state changed.

Bug: 74541410
Test: m -j ; make -j api-stubs-docs-update-current-api ; m -j ; atest android.media.cts.AudioManagerTest#testSpeakerphoneIntent
Change-Id: I01963493001e31b1edf1f1ba85b5744eb924d887