Fix accessibility issues in Private DNS Settings

- 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