Performe save when ENTER is pressed.

Push save enabling logic into SaveFragment, let it figure it out.
Don't assume DirectoryFragment is present in BaseActivity.
Permit DirectoryFragment#get to return null when not present.

Bug: 24305559
Bug: 24955068
Change-Id: Ib50d674b16ba48dcefb6e5b73055120c3e7c6dba