Add the MIN_UNDEQUEUED_BUFFERS query to ANW.

This change adds a new query to ANativeWindow for getting the minimum
number of buffers that must be left un-dequeued during the steady-state
operation of the ANativeWindow.

Change-Id: Ie8c461fc26b02ecde02ddb4f95bf763662cf1551
Related-Bug: 3356050
diff --git a/include/gui/SurfaceTexture.h b/include/gui/SurfaceTexture.h
index 31615d0..9bf38f7 100644
--- a/include/gui/SurfaceTexture.h
+++ b/include/gui/SurfaceTexture.h
@@ -37,7 +37,8 @@
 
 class SurfaceTexture : public BnSurfaceTexture {
 public:
-    enum { MIN_BUFFER_SLOTS = 3 };
+    enum { MIN_UNDEQUEUED_BUFFERS = 2 };
+    enum { MIN_BUFFER_SLOTS = MIN_UNDEQUEUED_BUFFERS + 1 };
     enum { NUM_BUFFER_SLOTS = 32 };
 
     struct FrameAvailableListener : public virtual RefBase {