Make Activity.getReferrer tolerate custom Parcelables in intents

Referrer information is passed as an intent extra to each Activity,
therefore if that intent contains a custom parcelable unmarshaling
the intent extras will fail in a call to getReferrer. Insulate apps
from this concern and log a warning when it happens.

Bug 28972438

Change-Id: I37dbc033bfcfe816e4b58b382c012854fbed943b