Enable microphone selection

Expose the device's microphones for testing purposes through the
HAL. One has to pass a specific parameter to the `AudioManager`,
namely 'fp_test'. Accepted values are:
* 'primary_mic'
* 'secondary_mic'
* any other value to disable the microphone selection and reset
  the default behaviour

Note that there is no safeguard check, if one selects a microphone,
the test mode will be entered and will only be left when the
`AudioManager` receiveds the third possible 'fp_test' value.

FPIIM-641

Change-Id: I291162ca42c200ff3bed62901f71aa84ebc5ccb6
2 files changed