Test: parcel marshalling for user credentials page

Unit test to verify that the internal Credential that gets passed to
dialogs for removing said credential is actually a valid Parcelable
object, since it's hard to provoke a marshal/unmarshal in real-world
usage (but not impossible).

Bug: 22541933
Change-Id: I780ca2d7b01fc6081b9ea8b2810cfc97f0433a86