Update ActivityId to implement Parcelable

This API change is safe because all constructors of ActivityId were
hidden, so we can mark this class as final and update it to implement
the Parcelable interface.

Test: atest CtsAssistTestCases:ActivityIdTest
Bug: 215589465
Change-Id: I4f233b4cf93e0bdd40ed371fad151e6fe6c104c7
3 files changed