Remove field when removing the only char

This allows us to make a simplifying assumption for b/18004959.
After this, we can assume that updateEmptyEditors is never called
with emptyEditors.size() > 1 and findFocus() == null.

Bug: 18004959
Bug: 18194655
Change-Id: Idaf2c64bf3d359c1aa8938150a6cda3684542d42