Don't get struct of children of !laidout viewGroup

Some view groups (e.g. the support.v4.viewPager) need to be laid out
to determine the order of it's children. As we need the order before
being able to collect the structure of the children we need to skip
their collection until they are laid out.

Fixes: 38198484
Test: cts-tradefed run cts-dev -m CtsAutoFillServiceTestCases
      cts-tradefed run cts-dev -m CtsAccessibilityTestCases
      cts-tradefed run cts-dev -m CtsAccessibilityServiceTestCases

Change-Id: I1f7bee1505edaf70c9d9a686be6f9a76d8aedd0b