[Android] Check folder existence before setting title in Edit activity.
When a folder is deleted in EnhancedBookmarkAddEditFolderActivity,
we listen to the bookmark model change and dismiss the activity if it
is deleted. However, at onStop(), we were always trying to save the
folder title, and if it was deleted already, it crashed.
Fix the crash by simply adding an existence check before we save the
title.
BUG=522046
Review URL: https://codereview.chromium.org/
1300843004
Cr-Commit-Position: refs/heads/master@{#343993}