audio hal: do not use direct output for VoIP use case

Using direct output for VoIP use case prevents applications from using more
than one audio track simultaneously.

Note: As audioflinger mixer does not support mono outputs, the output
stream is configured in stereo and stereo to mono conversion is
performed by the HAL.

Bug: 64392515
Test: Fi Calls and check signalling
Change-Id: Ibf4a7c7c41c96ed46cea9a1ded069ce612ea9a6d