Hearing Aid: Combine audio from stereo to mono when only one side connect

* On the design of the Hearing Aids, the phone will separate the stereo
audio samples and stream the left audio samples to the left device and
right audio samples to the right device. When only one side connected,
phone will only stream the one side audio sample to one hearing aid
device.
* This CL adds a functionality for checking the left/right hearing aid
and combine audio samples from stereo to mono when only on side connect.

Bug: 117178490
Test: disconnected/reconnected one side hearing when play stereo test
sound.

Change-Id: Idb3871951617680aba3ca82f801a70c95403d889