Fixed unsafe lock upon safe media volume

Safe media volume uses lock as Integer.
But it is not safe since the Integer value is changed.

disableSafeMediaVolume()
   synchronized (mSafeMediaVolumeState)  // step. 1
      setSafeMediaVolumeEnabled()
         mSafeMediaVolumeState = SAFE_MEDIA_VOLUME_ACTIVE  // step. 2
      ...
      onSetStreamVolume(mPendingVolumeCommand.mStreamType, // step.4
      -> mPendingVolumeCommand is set as null by step.3
      -> it causes NPE and reboot

------------

setStreamVolume()
   synchronized (mSafeMediaVolumeState)
   -> mSafeMediaVolumeState was changed by step.2
   -> so that it would go next step
      mPendingVolumeCommand = null;  // step. 3

Test: Build Pass, manual test, change volume
Change-Id: I33f473d42ccbf0f9b177c6886622ecc2f8020f8d