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
Change-Id: I86a5a15715cc6dde0eae93d38cd1c5cef33d7c67
19 files changed