Add FdFile::Compare

Also added unit test. Will be used in tests.

Bug: 63467744
Test: test-art-host-gtest-fd_file_test -j40

(cherry picked from commit 27d680adac56af90906970bfddb404bbb4342ee4)

Change-Id: I5fbfa467e5f3591eadd78905fd5aee7a7712c0b8
3 files changed