Remove fade manager aconfig from media solutions repo

The flag was initially defined in media_solutions namespace
whereas the correct namespace is media_audio.

Move the flag to the audio repo while maintaining the same
naming convention.

This commit removes the flag definition from media scope
and updates import to audio.

Bug: 186905459
Bug: 307354764
API-Coverage-Bug: 308666800
Test: atest -c FadeManagerConfigurationUnitTest

Change-Id: I4f8701330f5e61aa5601db99f51c1b963eaed107