SysUIG override for ShutdownUI.

Extract layout choice to separate method;
include generic ShutdownUi specifically in non-SysUIG
versons rather than in all SysUIs.

Bug: 267722480
Test: builds; no user visible changes in shutdown flow
Merged-In: Iab225a5b733a78c6e0f8f5395f1516fe765994e2
Change-Id: Iab225a5b733a78c6e0f8f5395f1516fe765994e2