DO NOT MERGE Add "dumpsys SurfaceFlinger --dispsync"

Dumps the current DispSync state.

Bug 14651879

(this is a near-cherrypick of Ide4e6dbd58b117bc1a6b97b57d10cd92ec86dc84)

Change-Id: I6e6c8452ede5c2d5098db1b884d28226e77d9a03
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 1a9a694..b1db870 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -2201,6 +2201,13 @@
                 clearStatsLocked(args, index, result);
                 dumpAll = false;
             }
+
+            if ((index < numArgs) &&
+                    (args[index] == String16("--dispsync"))) {
+                index++;
+                mPrimaryDispSync.dump(result);
+                dumpAll = false;
+            }
         }
 
         if (dumpAll) {