Stick FileOperationService to foreground.

Note there is still a small window between service is started and our
service enters foreground mode because originally we only show
notifications once job has started. Therefore our service is still a
background service before the first job starts to set up.

Bug: 34512210
Test: Automatic tests pass. Some manual tests.
Change-Id: I33987c1da861bb7583d12a21547f44c77de860d8