aaudio: convert mono output to stereo

MMAP hardware streams are often stereo.
But apps often want to output a mono stream.
Since converting from mono to stereo is easy,
we can go ahead and open a stereo hardware stream
and then just convert the apps mono data to stereo for the HW.

Add getDeviceChannelCount().

Test:  adb shell write_sine_callback -pl -s10 -c1 -m3
Test:  adb shell write_sine_callback -pl -s10 -c1 -m3 -x
Change-Id: I444a38c6f5cd32d1d6113f16aacec68285a1bc82