Overhaul RenderNode's DisplayList management

* Move mValid to native
* Have destroyHardwareResources destroy everything
* Remove flaky mParentCount checks in setStaging
* All tree updates have an internal observer to
  ensure onRemovedFromTree() is a reliable signal
* onRemovedFromTree() immediately releases resources
  to avoid displaylist "leaks"

Test: Unit tests for validity added & pass, manually
verified that b/34072929 doesn't repro

Bug: 34072929

Change-Id: I856534b4ed1b7f009fc4b7cd13209b97fa42a71c