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