Un-hide ViewGroup#onViewAdded/onViewRemoved

These methods are generally useful for writing custom views, and by
exposing them we make it easier for custom view authors to still allow
app developers to use an OnHierarchyChangedListener since it will not
be occupied by a custom view's implementation.

Also move the actual dispatch to package-scoped dispatch methods so
that a developer forgetting to call super won't stop a listener from
functioning.

Bug 21866523

Change-Id: Ie2bb5e241d7c5a02a5033f33ecdaeb40aceb20b5