AudioMix: Always produce output channel masks from Builder

Ensure that AudioMix.Builder always produces and AudioMix
which uses an output channel mask. There are assumptions
in the framework that this is the case, however clients
can actually pass an input mask.

Bug: 194910301
Test: gts-tradefed run gts -m GtsGmscoreHostTestCases \
      -t com.google.android.gts.audio.AudioHostTest
Change-Id: I698eef509a0e3427b7cfc4bf8b045379416f7964