hal: fix voice call device routing issue

adev->voice_device_set flag is to indicate voice call
device routing update from policymanager to HAL. It is
set to true in voice_start_call and reset in update_calls()
which causes mismatch in flag update during back to back
voice calls scenario. Update adev->voice_device_set flag
in voice_stop_call instead of update_calls(). Rename
voice_device_set flag to in_call for readability.

Change-Id: Ie07105671f254899890bdb4c0635c7dc1f55dbff
hal: NULL pointer check before using audio_usecase

- Add check for NULL pointer before using audio_usecase

Change-Id: Iaedd7b1dfbcf727957f6b19a54199d29afa25703