CSD: expose TestApi methods to test the feature

Exposing methods for getting/setting RS2 and CSD value. Also introducing
the possibility to force the internal MEL computation and to force CSD
for all output devices.
Removed unused mCurrentImeUid to unblock the presubmit.

Test: atest SoundDoseHelperTest
Bug: 248565894
Change-Id: Ic40d41597ca3a91b9e219192bbc01ea7f5afd084