Add focus method to BookletLayout

So callers can explicitly focus (or re-focus) the first
input in the current page. Logic was moved from onStackLayoutSet.

Also rename updateOutlineSelectWidget() to
selectFirstSelectablePage(), which better describes
what the method does and makes me feel less guilty
about calling it from focus().

Change-Id: I60377cb5bf0b104ee80e555ca18a8edfe7c0889e
Merge "Add focus method to BookletLayout"