Move showUserSwitcher to UserInteractor

This method will be needed by the status bar user chip, and has the
equivalent behavior that was defined in `FooterActionsInteractor`. This
CL moves the implementation + tests up from `FooterActionsInteractor` to
a new dependency on `UserInteractor`

Test: atest UserInteractorTest
Bug: 254246505
Change-Id: Ia78513a0f86a5ade8727855f677919abac82b3b1