Make mk2rbc output paths relative to android root

Previously, mk2rbc would output absolute paths in the load statements.
This caused issues with the new module resolution logic in rbcrun,
and is not something we'd want to check in anyways.

Bug: 280685526
Test: go test
Change-Id: Iae6e3f6be9cecc8e19b6e1fd33c5575ff6864654