DDM: Introduce STAG packets

This is a new mecanism to help DDM client synchronize with app
state.

Design doc: go/ddm-stag
Test: Android Studio
Bug: NA

Change-Id: I4699c62756f50106962ec8481bf67f103fa41b8f
diff --git a/Android.bp b/Android.bp
index effd7ce..3709d75 100644
--- a/Android.bp
+++ b/Android.bp
@@ -503,6 +503,13 @@
 }
 
 filegroup {
+    name: "framework-android-os-unit-testable-src",
+    srcs: [
+        "core/java/android/os/DdmSyncState.java",
+    ],
+}
+
+filegroup {
     name: "framework-networkstack-shared-srcs",
     srcs: [
         // TODO: remove these annotations as soon as we can use andoid.support.annotations.*