Move getUserSwitchability from UserManager to UserManagerService.

UserManager.getUserSwitchability was calling telecomManager.isInCall
method to see whether there is an ongoing phone call. Since the call
was made from client side, it was causing a SecurityException when
switcing users via adb command and breaking CtsMultiUserHostTestCases
tests. This CL moves all the logic from UserManager to
UserManagerService, and calls telecomManager.isInCall() with
Binder.clearCallingIdentity.

Bug: 257538981
Bug: 261505371
Test: atest CtsMultiUserHostTestCases
Change-Id: Idfca1fcb89688d08878b908ceed9c64f1e7091d3