Fix issue 2909189: System property ro.audio.silent no longer mutes system.

Fixed regression introduced by commit 2a6b80bc65c4782b5a7168b300e1dc5ec9f617ee:
master mute was not working if no effect chains were present on session 0.

Change-Id: I66d107e045d159cb94d29c7476fa1e12d92f2ae7
diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index 1c7faa4..6e7633e 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -1675,6 +1675,9 @@
     float masterVolume = mMasterVolume;
     bool  masterMute = mMasterMute;
 
+    if (masterMute) {
+        masterVolume = 0;
+    }
 #ifdef LVMX
     bool tracksConnectedChanged = false;
     bool stateChanged = false;
@@ -1696,10 +1699,7 @@
     // Delegate master volume control to effect in output mix effect chain if needed
     sp<EffectChain> chain = getEffectChain_l(AudioSystem::SESSION_OUTPUT_MIX);
     if (chain != 0) {
-        uint32_t v = 0;
-        if (!masterMute) {
-            v = (uint32_t)(masterVolume * (1 << 24));
-        }
+        uint32_t v = (uint32_t)(masterVolume * (1 << 24));
         chain->setVolume_l(&v, &v);
         masterVolume = (float)((v + (1 << 23)) >> 24);
         chain.clear();