Preference multichannel pcm direct output over stereo mixer

If a direct output is available for a multichannel output request,
and the mixer outputs a channel count lower than the track, try the
direct output first, and fall-back to the non-direct output if
not successful.

Bug: 238286324
Test: atest audiopolicy_tests:AudioPolicyManagerCarTest#GetOutputForAttrAtmosOutputAfterRegisteringPolicyMix
Change-Id: Ic7265bd09556ab7dd0660df5b307d151abace503