Editor: Handle content uri's in the editor

Previously, files opened that were not contained
inside the /data/media partition were considered
invalid.

Repo steps:
- Open email with .txt attachment
- Open attachment with Editor

This also fixes similar issues all across the
system where content uri's would not open in
editor. Previously they would report that the
user needed to gain root access.

Change-Id: I15aa3fd81ae261f66920d253d6adebe28cf28e66
(cherry picked from commit e09d8433)