Guest mode updates to resolve privacy concerns in guest mode

- Add API in IUserManager to allow setting ephemeral user flag
- Implement and export this API in UserManagerService and UserManager
- Set guest as ephermal by default when createGuest in UserManager is called
- Handle guest user switching in UserSwitcherController for the case
  of dynamic change of ephemeral state
- Add persistant notification when in guest mode to indicate
  - if guest session is new or previously used.
  - if guest session will be cleared on exit or not
- Add buttons in persistant notification to reset or exit guest
- Add flags to enable/disable this feature

Bug: 214031645
Screenshots: go/ephemeral-guest-b-214031645-ux
Test: Manual test using sunfish, atest SystemUITests, atest SettingsRoboTests

Relands ag/16545010 after resolving post submit issues

Revert "Revert "Guest mode updates to resolve privacy concerns in guest mode""
This reverts commit dd5c4408.

Change-Id: I46b8ab527bab8fe665114ed0fffbb06a59d49a77
Merged-In: I46b8ab527bab8fe665114ed0fffbb06a59d49a77