Fix FDN validity check UX.

Use a text watcher to enable/disable the save button instead of showing
a toast that the save button should not be used at that point.

Change-Id: Iaf5f9c9e7a53b3184bb1423d45f80948bfc8ebe0
Add missing space.

Change-Id: Ibd4b648012dd473468598355b5a58f7309102bd4