audio: Restrict reported channel masks to "mono" and "stereo"

When translating supported channel counts into channel masks,
only use channel layouts for 1 and 2 channels: "mono" and
"stereo", while reporting all other counts via indexed
masks. This helps to support output to multichannel USB Audio
interfaces properly.

Bug: 329571487
Test: repro steps in the bug
Change-Id: Ibbbf71e7f04a6622cccaddeb3f4f140193d7f8fb