Use iterator to remove elements from collection when traversing it

Per Java documentation "Iterator.remove is the only safe way to
modify a collection during iteration; the behavior is unspecified
if the underlying collection is modified in any other way while
the iteration is in progress."

Test: updated gmscore to check that onPackageRemoved() works correctly
Bug: 35846264
Change-Id: I878e581db3bf60b9c6b2cf861f66d8b427202b02