Animation fixes when task is not resumed

- Make sure to add the activity to mWaitingVisibleActivities
to not destroy the activity before the app transition has
started.
- Defer layouts in startActivityUnchecked so when clearing the
current activities in the current stack, we don't execute the
app transition yet. We need to wait until the new activities
are added as well.

Bug: 28026847
Change-Id: I252cf139e06197ea34329d1466f1f6162f3e7a30