Exempt RRO from the restriction on API level

As auto generated RRO, an arbitrary RRO needs to be exempted as well.
It is okay because RRO packages can access only resource defined in
public.xml, when RRO packages want to refer other package's resource.

Bug: 152926556
Test: build RRO packages
Test: check if product/vendor app still can't use hidden APIs
Merged-In: I8dc2922af5eb5c87ccd97d1b8d080b3ed48cf9a6
Change-Id: Ief10b3535d4eb9f6607cac31cba1b4d3e081efd0
(cherry picked from commit 07a44f48)