Add an on-screen overdraw counter

The counter can be enabled by setting the system property called
debug.hwui.overdraw to the string "count". If the string is set
to "show", overdraw will be highlighted on screen instead of
printing out a simple counter.

Change-Id: I9a9c970d54bffab43138bbb7682f6c04bc2c40bd
diff --git a/libs/hwui/Properties.h b/libs/hwui/Properties.h
index 87ac845..7c68b5b 100644
--- a/libs/hwui/Properties.h
+++ b/libs/hwui/Properties.h
@@ -71,9 +71,9 @@
 
 /**
  * Used to enable/disable overdraw debugging. The accepted values are
- * "true" and "false". The default value is "false".
+ * "show", "count" and "false". The default value is "false".
  */
-#define PROPERTY_DEBUG_OVERDRAW "debug.hwui.show_overdraw"
+#define PROPERTY_DEBUG_OVERDRAW "debug.hwui.overdraw"
 
 /**
  * Used to enable/disable non-rectangular clipping debugging.