surfaceflinger: call into hardware composer to blank screen

Call into the new HWC blank operation to blank or unblank the screen.
Legacy systems may have already blanked the screen via early suspend,
and can choose to not implement the blank operation in their hardware
composer implementation.

Change-Id: Ib403c8c0e36367a2cfef3e1d124872fcfeb9e7cb
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index a662b25..5cde7fb 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -71,9 +71,12 @@
     // commits the list
     status_t commit() const;
 
-    // release hardware resources
+    // release hardware resources and blank screen
     status_t release() const;
 
+    // acquire hardware resources and unblank screen
+    status_t acquire() const;
+
     // create a work list for numLayers layer. sets HWC_GEOMETRY_CHANGED.
     status_t createWorkList(size_t numLayers);