Set root dir in lint project.xml

Set the root dir in the project.xml for lint so that the paths in the
lint report are relative to the top of the tree instead of relative
to the project.xml.

Bug: 153485543
Test: run lint
Change-Id: Ie163a4dadd976e708f798855de73e58084931a91
Merged-In: Ie163a4dadd976e708f798855de73e58084931a91
(cherry picked from commit c31efeb2)