FastMixer: Enable Balance by setting channel mask

Ensure Balance channel mask is set if onStateChange not called,
otherwise it doesn't do anything.

Test: Play low latency track, adjust balance left/right
Bug: 203617292
Change-Id: I7da74b2e7c45167b38115c8591aea7164d6d790a
diff --git a/services/audioflinger/FastMixer.cpp b/services/audioflinger/FastMixer.cpp
index fc34d95..26bd92d 100644
--- a/services/audioflinger/FastMixer.cpp
+++ b/services/audioflinger/FastMixer.cpp
@@ -89,6 +89,7 @@
     // TODO: Add channel mask to NBAIO_Format.
     // We assume that the channel mask must be a valid positional channel mask.
     mSinkChannelMask = getChannelMaskFromCount(mSinkChannelCount);
+    mBalance.setChannelMask(mSinkChannelMask);
 
     unsigned i;
     for (i = 0; i < FastMixerState::sMaxFastTracks; ++i) {