Support excluding overlays when enforcing RRO

PRODUCT_ENFORCE_RRO_EXCLUDED_OVERLAYS will be used to specify overlays
to be excluded from enforcing RRO.

The excluded overlays will be applied into the original package.

Bug: 63600240
Bug: 65001751
Test: succeeded building with
PRODUCT_ENFORCE_RRO_EXCLUDED_OVERLAYS := \
    vendor/google/google_overlay/static_only \
    vendor/google/nexus_overlay/common/static_only
and confirmed that config_webview_packages existed in framework-res.apk
with the overlayed value, but not in framework-res__auto_generated_rro.apk.

Change-Id: I0bfb44fc7726710bb78d9100404bc6dd29d06a73
diff --git a/core/product.mk b/core/product.mk
index 1a0e8c6..95e3b27 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -94,6 +94,7 @@
     PRODUCT_EXTRA_RECOVERY_KEYS \
     PRODUCT_PACKAGE_OVERLAYS \
     DEVICE_PACKAGE_OVERLAYS \
+    PRODUCT_ENFORCE_RRO_EXCLUDED_OVERLAYS \
     PRODUCT_ENFORCE_RRO_TARGETS \
     PRODUCT_SDK_ATREE_FILES \
     PRODUCT_SDK_ADDON_NAME \