Add fade manager aconfig to media audio

The flag was initially part of 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 adds the flag definition to media scope and
makes it accessible to car framework.

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

Change-Id: I240826380a7bf1f1e70348767877f50ea173808b