GlobalActions: Create a separate module for ShutdownUI.

Existing GlobalActionsImpl will delegate to ShutdownUi, which is
currently exactly the same implementation.

This allows us to override ShutdownUi behavior elsewhere, although in
this CL we use the same implementation in all SysUIs.

Bug: 246980844
Test: builds, boots; no user visible change to shutdown

Change-Id: Icbf04b5f98b891ca43041ae6e8637a1e03020ec0