audio policy: fix beacon mute delay

Only take output latency into account when the output is active
when muting TTS stream. This avoid a systematic long delay the
first time we play audio due to first beacon mute state change.

Bug: 37923452
Test: repro steps in bug
Change-Id: I10b21a55c0fad28344108b67756f600a29e41fea
1 file changed