set flag on startup to use in checking for Android
On android we access photos directly on the file system rather than in
their own app sandbox as on iOS so we need to copy files instead of
moving them as otherwise we end up deleting people's photos which would
be bad. As part of this we should avoid trying to copy or move a file on
to itself as that fails so check that the source and destination files
aren't the same. This should sort out the problems in #43.