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/libs/surfaceflinger_client/Surface.cpp b/libs/surfaceflinger_client/Surface.cpp
index 1e9bd74..2c77edf 100644
--- a/libs/surfaceflinger_client/Surface.cpp
+++ b/libs/surfaceflinger_client/Surface.cpp
@@ -709,6 +709,9 @@
     case NATIVE_WINDOW_FORMAT:
         *value = int(mFormat);
         return NO_ERROR;
+    case NATIVE_WINDOW_MIN_UNDEQUEUED_BUFFERS:
+        *value = MIN_UNDEQUEUED_BUFFERS;
+        return NO_ERROR;
     }
     return BAD_VALUE;
 }