policy: Delay FM volume update to start of FM playback

Delay update of FM volume to start of FM playback.
This avoids unnecessary set_parameter calls on primary output
whenever volume changes on primary output. Unnecessary calls
on primary output can cause breaks in audio since FastMixer
will be put to HOT_IDLE more often

CRs-Fixed: 948361
Change-Id: Id614d32df7f35ac93dd029a08b10e19f64ac631e
diff --git a/policy_hal/AudioPolicyManager.h b/policy_hal/AudioPolicyManager.h
index 59124b8..af8c2a8 100644
--- a/policy_hal/AudioPolicyManager.h
+++ b/policy_hal/AudioPolicyManager.h
@@ -155,6 +155,7 @@
         bool mIsInputRequestOnProgress;
 #endif
         float mPrevFMVolumeDb;
+        bool mFMIsActive;
 };
 
 };