AudioManager: add method to query stream attenuations in dB

AudioManager: new methods in SDK for querying:
 - the minimum volume index
 - the volume in dB for a given index on a given device type

Bug: 64844598
Test: cts-tradefed run cts-dev -m CtsMediaTestCases -t android.media.cts.AudioManagerTest
Change-Id: Iac999e7676643272820a4f1f4a32cf3a781ab7cd