Do not cache bag parent stack until requested

Bag parent stacks are used when inspecting/debugging layouts. Since
they are not needed at runtime when running applications normally,
do not cache bag the parent stack when calling AssetManager::GetBag.

Bug: none
Test: libandroidfw_tests
Change-Id: I29ccdc8ed30d4f2d11ee51ef58f6e459ca6c1f6a