audio: Fix remote submix configuration and 'prepareToClose'

In the configuration, remove "mono" channel masks.
The legacy implementation always used stereo, the framework
handles channel conversion. Otherwise, the input may
be opened with a different channel mask from the output.

In 'prepareToClose', close the output side of the route.
The framework may close the "old" output stream after
it has opened a new one. A call to 'prepareToClose' means
that the framework thread is already shutting down, thus
it is safe to shut down the route.

Bug: 302036943
Test: atest audioeffect_analysis
Change-Id: I95b57807f2559ef681da822b3cab4ea6b4340143