Don't copy a directory into itself...doesn't go well.

Minimally deform CopyService such that we can listen
    to the completion of operations in the test.
Add test coverage.
Add equals and hashcode to DocumentInfo...so we can compare
    the heck out of 'em. + a test.
WIP: Expose (@hide style) DocumentsProvider.isChildDocument via
    DocumentsContract. Use that to check for recusive copies.

Bug: 25794511

Change-Id: I05bb329eb10b43540c6806d634e5b96a753e8178