Sanitize display names, keep extensions intact.

When creating or renaming files on external storage, sanitize the
requested display names to be valid FAT filenames.  This also fixes
a handful of directory traversal bugs.

Also relax logic around generating display names to allow any
extension which maps to the requested MIME type.  Tests to verify.

Bug: 18512473, 18504132
Change-Id: I89e632019ee145f53d9d9d2050932f8939a756af
(cherry picked from commit 385e95f8)