audio: Simplify and extend alsa::Mixer

Remove alsa::MixerControl. tinyALSA contains utility
functions for setting values in percents, they use
the same logic as used to be there for handling
the "volume" control. Use access serialization at
the mixer level, rather than for each control.

Move the call to 'mixer_open' to alsa::Mixer.

Add controls for capture (mic) mute and gain. They
will be used by the primary HAL.

Bug: 264712385
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I0fad994153de96aceec3eb8f2fec19805ec912f8