AudioManager: isFixedVolume() fix for dynamic volume behavior

AudioManager.isFixedVolume() was only considering the fixed volume
global property, but was not checking the current volume behavior.

Bug: 203034282
Test: atest MediaActivityTest ; atest MediaRouter2Test#testCallbacksAreCalledWhenVolumeChanged
Change-Id: I1e22e5bcd75c41c1ab6a47dc1e0d7619df989970