New LOCAL_SDK_RES_VERSION config var.

This allows an app to target a different SDK level for its resources
than it uses for its Java code.  This is useful because it is generally
safe to use various newer symbols like attribute names, as older versions
of the platform will simply ignore them.

Change-Id: Ida19ec9f259c4ea70de846e9a63546e263b1c91e
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c5e4cc7..4e6858f 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -93,6 +93,7 @@
 LOCAL_DX_FLAGS:=
 LOCAL_CERTIFICATE:=
 LOCAL_SDK_VERSION:=
+LOCAL_SDK_RES_VERSION:=
 LOCAL_NDK_VERSION:=
 LOCAL_NDK_STL_VARIANT:=
 LOCAL_NO_EMMA_INSTRUMENT:=