Update UI for factory reset flow

After applying a collapsing toolbar in the Settings app, the factory
reset screen is not displayed correctly. This is because
1) the factory reset page should not essentially have a toolbar since it
has different UI layout form other pages.
2) this page was launched previously as a SubSettings so it applied a
wrong theme.

This change is mainly updating the inheritence structure and specifying
the right theme for the factory reset page in order to have a correct
layout.

Bug: 179457252
Test: robotests and visual verified
Change-Id: I91aec65a120d5a288507d89ace7c9298acdca42b