Guest mode UX flow updates to user settings

- Add option in user settings to enable/disable ephemeral
  mode for guest user
- Update user settings to show exit guest and reset guest preferences
- Update user settings to show guest related preferences grouped together

Bug: 214031645, 175795666
Screenshots: go/ephemeral-guest-b-214031645-ux
Test: Manual test on sunfish, atest SystemUITests, atest SettingsRoboTests
Change-Id: I7cf205883f8250178ac24c67c74b9142ceb6c1f5