LT-14067

There were two problems to solve in this JIRA issue:
1. Elsewhere the phonological feature chooser used "<n/a>"
   to mean "remove this value".  Here it does not mean that.
2. If one set a value or "Remove this feature" in the Change to
   combo and then used the phonological feature chooser (but did
   not select anything or canceled out of it), what showed in the
   combo gave incorrect information.

These code changes fix these two items:
1. We use "<ignore>" instead of "<n/a>" for this usage of the
   phonological feature chooser.
2. Whenever a user cancels out of the phonological feature chooser
   or does not select anything, we show the last selected value.

Change-Id: If7ad1b242a0d9515e24dc6db7c10e4ef1640957d