ART: Add Unlink to FdFile

Add Unlink function that tries to unlink the file if it
was created with a file path. The function tries to
ensure that it does not unlink a newer file.

Add a parameter to Erase to add unlinking.

Test: m test-art-host-gtest-fd_file_test
Change-Id: I49993bb94aec10d5c8d9b2cbea30ebaa255b99e1