hwc: Rename two app buffer property.

Read only property will not work form HAL,
hence renaming it.

Change-Id: I7be4014908a955bb69e714556622d06c09318afa
diff --git a/libhwcomposer/hwc_utils.cpp b/libhwcomposer/hwc_utils.cpp
index 8139013..6a52388 100644
--- a/libhwcomposer/hwc_utils.cpp
+++ b/libhwcomposer/hwc_utils.cpp
@@ -74,7 +74,7 @@
 #endif
 #endif
 
-#define PROP_DEFAULT_APPBUFFER  "ro.sf.default_app_buffer"
+#define PROP_DEFAULT_APPBUFFER  "sf.default.app_buffer_count"
 #define MAX_RAM_SIZE  512*1024*1024
 #define qHD_WIDTH 540