Added UserManager#getUserSwitchability.

The new SystemApi indicates whether user switching is currently allowed
or not. This API provides more detail than the already existing
UserManager#canSwitchUsers (now deprecated) by telling the user why
users cannot be switched via a flag that's returned.

Bug: 124883455
Test: atest android.server.pm.UserManagerTest#testGetUserSwitchability
Change-Id: Ib3ac2f063f3638a172fa339fd2330604b910c7f4