Use LiveData for adding a role holder in role request.

Due to the async nature of role operations, we need an object to take
care of the RoleManager.addRoleHolderAsUser() callback, keep track of
the state while avoiding leaking a surrounding context. ViewModel and
LiveData is a good fit for this scenario.

Bug: 110557011
Test: build
Change-Id: Id6ad1c92d7046ecfbc232ef0133b0566d4f7750c