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

Relands ag/16544951 after fixing post submit issues

Revert "Revert "Guest mode UX flow updates to user settings""

This reverts commit ed45e8c5.

Change-Id: I54583f9021171ae523ff40d4f63835f1cb486e35