Make the MIDI Manager optional, enabled by "android.software.midi" feature

Change-Id: I76d442ea28beea4b9e2876bfef501d8f61403702
diff --git a/services/Android.mk b/services/Android.mk
index 8777085..1918db5 100644
--- a/services/Android.mk
+++ b/services/Android.mk
@@ -24,6 +24,7 @@
     appwidget \
     backup \
     devicepolicy \
+    midi \
     net \
     print \
     restrictions \