Refactor surfacefinger latchBuffer: Create method

Shortens latchBuffer method, extracting transaction
credential verification to dedicated function.

Test: On device
Change-Id: Ia7c2f977a85d1c1afd87425588c07eb756b00f15
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index efb0fb8..64b049c 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -558,6 +558,11 @@
 
     // -----------------------------------------------------------------------
 
+    // Check all of the local sync points to ensure that all transactions
+    // which need to have been applied prior to the frame which is about to
+    // be latched have signaled
+    bool allTransactionsSignaled();
+
     // constants
     sp<SurfaceFlingerConsumer> mSurfaceFlingerConsumer;
     sp<IGraphicBufferProducer> mProducer;