UserManager.setUserName: clearCallingId pre-Broadcast

UserManagerService.setUserName sends a broadcast to all users.
Without doing clearCallingIdentity first, it would require
INTERACT_ACROSS_USERS permission, which is not required for
this function.

Bug: 128576953
Test: Manual
Change-Id: Ia577248f48234eb9b73e53f9f9da04a86c324e12