audio: only allow AudioMix with same format on direct output.

When the AudioMix rule is match the direct output and both format are
pcm, the AudioPolicy is going to invalidate the AudioTrack and it
will cause audio cutting on direct output.
For AudioTrack going to direct output, AudioMix should only be
permitted when the format is exactly same.

Bug: 326571112
Test: Manual
Change-Id: I47bea6c4695a977d38a5d95e155b782a150bfaaa