Get filename from File faster [DO NOT MERGE]

Use the direct call from a File object to get the filename instead of
going through Path which depends on Java 7 making the LayoutLib tests
incompatible with Java 6.

Change-Id: I815895eedbc10245ee09bdb53b11e5548b076aad
(cherry picked from commit 7dbf91fee96790fbeb9858740be1bb375f29ecb5)