Allow excluding specific overlay sub-directories

This change allows PRODUCT_ENFORCE_RRO_EXCLUDED_OVERLAYS to
refer to subdirectories of directories in PRODUCT_PACKAGE_OVERLAYS,
e.g.:
PRODUCT_PACKAGE_OVERLAYS := foo/overlay
PRODUCT_ENFORCE_RRO_EXCLUDED_OVERLAYS := foo/overlay/packages/apps/HelloWorld

Bug: 118823052
Test: m nothing (soong tests)
Change-Id: Ic9f89c11d023ea1b4f6f2f6683d94c81aa7b8a1b