More fixes for VectorDrawable memory management

Make the release of all the VGroups deterministic once the root group
has been freed by a finalize call.
Also, free and clear the children array.

Disable assert in DelegateManager that would slow down layoutlib when
there are many delegates and assertions are enabled (dev and canary
builds).

Bug: http://b.android.com/214026
Change-Id: Ic7775d360ae4997f54f30fb75851879acaf8d824
(cherry picked from commit f6d8bba638baedc39d1d8f7bd3c69f4956819c71)