aaudio: fix multichannel legacy streams

Two fixes for multichannel playback over USB from
AAudio legacy streams:
1: AAudio use a channel index mask and not a positional mask
when channel count is greater than 2.
2: audio policy gives priority to an output mixer which can support
the requested channel mask without downmixing

Bug: 131685020
Test: play multichannel audio over USB
Change-Id: I13f2da48d47fcf81f522096d90ba59a89379cffb