AudioPolicyMix: fix loopback+render mix match rules

Fix matching logic for loopback+render rule and usage VOICE_COMMUNICATION.
Previous logic was requesting flag mAllowPrivilegedPlaybackCapture to
allow capturing playback with usage VOICE_COMMUNICATION.
This is not a requirement: only flag mVoiceCommunicationCaptureAllowed
together with a client not having opted out from capture by the system
is needed for a match.
Also rename mAllowPrivilegedPlaybackCapture to mAllowPrivilegedMediaPlaybackCapture
in AudioMix class to reflect better its actual role.

Test: atest AudioHostTest
Test: atest AudioPlaybackCaptureTest
Bug: 161569476
Change-Id: Iad0b7e89e68c9f2e59fabb75071a6425e9a29085