Make TetherSettings extend RestrictedDashboardFragment

RestrictedSettingsFragment is deprecated and no longer supported. To be
compatible with catalyst, change the super class to
RestrictedDashboardFragment.

Bug: 368359963
Test: atest TetherSettingsTest, manual test with the user restriction
Flag: EXEMPT N/A
Change-Id: If8c5af9d716c63fd33cb508394e7d13450c9d2e7