audio flinger: pass mixer configuration when opening an audio output

Add the possibility for audio policy manager to indicate a mixer
audio configuration (channel mask, format, sampling rate)
different from the audio HAL stream configuration when opening an
output stream with IAudioflinger::openOutput().

Bug: 188502620
Test: atest audiopolicy_tests

Change-Id: I0b4eaa9cd3e5574b75e3b914c854f97c104ca07e