Record provision of custom Intents in AssistContent

Simplifies behavior wrt automatically setting the
web URI for ACTION_VIEW Intents, by not doing so
for custom Intents.

Adds a boolean isAppProvidedIntent() as a signal for
recepients of the AssistContent that a custom Intent
was provided. Custom Intents are more likely to be
relevant.

Change-Id: Ibe4bfa040eba904411b9820ab4ddfcf509413829