Migrate VpnSettings from RestrictedSettingsFragment to RestrictedDashboardFragment.

This is required for adding an overlay vpn_settings2.xml for Tips
Button.

Test: VpnSettingsTest
Bug: 293213974
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f9ca569249331a062e6f58a49bb0bb53aa69b3db)
Merged-In: I26531afe75215c9531c0122d2e86fd5d0da4d3db
Change-Id: I26531afe75215c9531c0122d2e86fd5d0da4d3db