Make widget sheet layout container not focusable

SpringRelativeLayout views in widgets sheets UI are non-actionable
containers, and as such they would ideally not receive focus. They were
made focusable as a workaround for an issue where back action was no-op
when widget search text box was focused, and there were no other
focusable elements in the UI b/197129170.
WidgetSearchBarController handles back action by clearing
focus, which ends up refocusing the search text box. This CL, removes
this back action override, and let's back action through - it will end
up being handled by `WidgetsFullSheet.onBackInvoked` which resets the
search.

Bug: 365120544
Test: Manual - verified back action with widget search active (and in
      case with no results) in emulator
Flag: EXEMPT bugfix

Change-Id: I1fed37372521947b7d4644047807dc451b0119a4