ANativeWindow: Add NATIVE_WINDOW_BUFFER_AGE query

Adds a NATIVE_WINDOW_BUFFER_AGE query, which returns the age of the
contents of the most recently dequeued buffer as the number of frames
that have elapsed since it was last queued.

Change-Id: Ib6fd62945cb62d1e60133a65beee510363218a23

include/system/window.h

@@ -267,7 +267,16 @@ enum {
      * The default data space for the buffers as set by the consumer.
      * The values are defined in graphics.h.
      */
-    NATIVE_WINDOW_DEFAULT_DATASPACE = 12
+    NATIVE_WINDOW_DEFAULT_DATASPACE = 12,
+
+    /*
+     * Returns the age of the contents of the most recently dequeued buffer as
+     * the number of frames that have elapsed since it was last queued. For
+     * example, if the window is double-buffered, the age of any given buffer in
+     * steady state will be 2. If the dequeued buffer has never been queued, its
+     * age will be 0.
+     */
+    NATIVE_WINDOW_BUFFER_AGE = 13,
 };
 
 /* Valid operations for the (*perform)() hook.