Change name of overlay subdir 'sku'->'theme'

The name 'sku' does not fit well as we recommend to use the same
value for different SKUs if vendors wish to have the same colors
on those SKUs.

Bug: 32268656
Change-Id: Ibb29279343058724808e27ddf2be0d0da36b1312
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index fdaa624..f7567b4 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -364,7 +364,7 @@
 LOCAL_INIT_RC_64:=
 LOCAL_JAVA_LANGUAGE_VERSION:=
 LOCAL_IS_RUNTIME_RESOURCE_OVERLAY:=
-LOCAL_RRO_SKU:=
+LOCAL_RRO_THEME:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.