Set ApkAssets path for fabricated rros

Fabricated RROs do not provide assets but are stored on disk. Ensure
that the path to the frro is returned when querying for the ApkAssets
path (which is mostly for debug purposes).

Bug: 181338216
Test: enable frro, use cmd overlay lookup to see path in resolution
Change-Id: Ibf9b1bf0a995325affbf084c71b1e87c5682e734