audio policy: fix notification + voicemail routing

Consider an active VOICE_CALL stream as an indication
that notification routing and volume should match voice call routing.

Also limit notification volume to match current VOICE_CALL volume index,
not just VOICE_CALL volume curve with notification volume index.

Bug: 74019016
Test: manual: receive notification while listening to voicemail and
check volume and routing.

Change-Id: Ia9309bdb10b42f6a4fa8745daf3129c90d44f0b8