Move startUserInBackground to standalone and avoid user from being stopped unnecessarily

- Removed START_USER_IN_BACKGROUND in createAndMaangeUser
- Added startUserInBackground that can return whether user is started. It checks for whether more users can be started without stopping existing users.
- Added canStartMoreUsers in UserController and ActivityManagerService
- Updated javadoc of a few user management API in DevicePolicyManager
- In UserController.startUser, return false if maximum running user limit is reached when starting user in background
- Only stop guest or ephemeral user that is being switched out in stopGuestOrEphemeralUserIfBackground

Bug: 71694116
Test: Create 3 ephemeral users, can startUserInBackground for first two but failed for the third.
Test: Switch to first user, second user is not affected.
Test: Switch out first user, second and third user is not affected. Can startUserInBackground for third user at this point.
Change-Id: I46aa1d8788851b10b5b169ac656cb982791de479