AudioFlinger: Add master volume to the dumpsys

Now PlaybackThreads shows master volume, which enables tracking
subtle silence issues. (Note MmapThreads already dump master volume.)

Test: adb shell dumpsys media.audio_flinger
Bug: 145107983
Change-Id: I2efc1a133c0ea07929c646b7e7148a9f3f311912

services/audioflinger/Threads.cpp

@@ -2012,6 +2012,7 @@ void AudioFlinger::PlaybackThread::dumpTracks_l(int fd, const Vector<String16>&
 
 void AudioFlinger::PlaybackThread::dumpInternals_l(int fd, const Vector<String16>& args __unused)
 {
+    dprintf(fd, "  Master volume: %f\n", mMasterVolume);
     dprintf(fd, "  Master mute: %s\n", mMasterMute ? "on" : "off");
     if (mHapticChannelMask != AUDIO_CHANNEL_NONE) {
         dprintf(fd, "  Haptic channel mask: %#x (%s)\n", mHapticChannelMask,