Nest SystemUI alert dialogs in NestedScrollLayout

That way, they can be scrolled if they don't fit the screen (in
particular in landscape).

It's important to nest the whole layout and not parts of it, so as not
to modify the expected layout from AlertDialogLayout.

Test: manual
Fixes: 222683712
Change-Id: I5b23acaee70b1a5b5a14e845bd0de5ac17b761d1