Fix TetherSettings crash issue

- When the user is a guest user, the UI will remove all preferences to restrict setting changes. If the ViewModel updates the UI in this situation, it will cause Settings to crash.
  - Avoid to setup ViewModel when UI is restricted

Bug: 284435378
Test: Manual test
atest -c TetherSettingsTest

Change-Id: I52d4ea717c34eacc9cc2321e3950dc89408049f8
2 files changed