- Keep the Save button enabled at all times
- Show error in the Hostname view to remind the user
- "The field is required" error
- "The hostname you typed isn't valid" error
Bug: 386323822
Flag: EXEMPT bugfix
Test: Manual testing
atest -c PrivateDnsModeDialogPreferenceTest \
PrivateDnsPreferenceControllerTest
Change-Id: I63973bd5001b838d7f27827e6a6d4ac96ac78ca9