Handle the case where no spell checker is selected.

This follows up to a previous CL [1] that fixed Bug 26685795 that the
Settings app crashes when no spell checker is selected, but introduced
another Bug 26686710 that the user cannot re-select the spell checker
once the device has fallen into that state because the button to select
new spell checker is disabled.

This CL tries to deal with such a case more carefully and gracefully, by
adding more null checking and by showing a meaningful text
("Not selected" for English) to users when no spell checker is currently
selected.

  [1]: I65e6d269572e064aa6897807b6611ef947d90211
       093a6467

Bug: 26108333
Bug: 26685795
Bug: 26686710
Change-Id: I0ed71bbb580e3547d97e321799ac2b77b1f284a3