audio flinger: fix standby on output with HW A/V sync

Fix a bug in audio HAL pause logic on output threads with
HW A/V sync preventing the HAL to enter standby when
the audio track is stopped and detroyed.

Bug: 19980184.

Change-Id: Ia497dad23159038b447fcbc18a67bb61b70b79cc
diff --git a/services/audioflinger/Tracks.cpp b/services/audioflinger/Tracks.cpp
index 7692315..dc9f249 100644
--- a/services/audioflinger/Tracks.cpp
+++ b/services/audioflinger/Tracks.cpp
@@ -745,6 +745,7 @@
                 // move to STOPPING_2 when drain completes and then STOPPED
                 mState = STOPPING_1;
             }
+            playbackThread->broadcast_l();
             ALOGV("not stopping/stopped => stopping/stopped (%d) on thread %p", mName,
                     playbackThread);
         }