Remove RecentsCreateFragment, rename RecentsProvider.

RecentsProvider was poorly named, given that it has nothing to do with
"Recents" root. I renamed it so, although a bit wordy, at least decouple
any assumptions of its relationship with Recents root.

Also stripped out RecentsCreateFragment, and now make
CREATE/OPEN_TREE/PICK_COPY_DESTINATION open to default root if last
accessed directory does not exist.

Finally, reverted ag/875528 because I don't think that applies anymore.

Bug: 27307743
Change-Id: I83ed9cc45308f881a58f02beb95922f9b79b3658