Fix broken test testImIntentCustom

Decouple the logic that generates the custom IM intent
from the logic that determines whether or not to actually
assign the intent to the entry (based on whether or not the
intent is registered), so that we can test if the intent is
correctly generated even if there is no application installed
that can handle that custom IM protocol.
Change-Id: If5ce0cd7639a6c05bc89777684cdac45cd7bb6cd