Fix NPE with display lists when view not attached

There was logic in ViewGroup that assumed that an accelerated
view must always be able to get a display list for any child
that it was drawing. One situation occurred, however, that
caused a problem with this - a contacts activity was started
and not yet attached, but was being asked to render into an
accelerated canvas. We assumed that the child would have a display
list and simply called getDisplayList(). But since that call
returned null, we later deref'd the null object.

The fix is to check whether a child can have a display list
instead of assuming that it can just because the container view
is accelerated.

Change-Id: I7de62fd597ad50720c9585d621bec02e77c171df