signal mFrameAvailableCondition when AudioSource is stopped

This allows last read() to exit properly, otherwise it could get
stuck waiting for the condition forever.

Bug: 15849309
Change-Id: I79bba53630e59783a5fc041cfc5d3fdb2545cc2c
diff --git a/media/libstagefright/AudioSource.cpp b/media/libstagefright/AudioSource.cpp
index d9aed01..a67fabe 100644
--- a/media/libstagefright/AudioSource.cpp
+++ b/media/libstagefright/AudioSource.cpp
@@ -159,6 +159,8 @@
     waitOutstandingEncodingFrames_l();
     releaseQueuedFrames_l();
 
+    mFrameAvailableCondition.signal();
+
     return OK;
 }