Expose the dynamic routing enabled flag to CTS test

CarAudioManager behaves differently depends on whether the
dynamic routing flag is turned on of not. Therefore, a new @TestApi is
added to CarAudioManager for testing purpose.

Bug: 120627682
Test: atest android.car.cts.CarAudioManagerTest
Change-Id: I3701c738565213def895bb0307d82ab7405e7c0d
4 files changed