Always use the largeIcon as avatar replacement

A few apps were targeting >P but not providing avatars yet.
Even though this was an app bug it lead to some weirdness in
our UI. We therefore always do the workaround of using
the largeicon for an avatar instead of generating one if we're
in a OneToOne conversation.

Bug: 153466270
Test: post hangouts message, observe normal person avatar
Change-Id: Ia5262ca7dfc20d265c2d0532e88932c878f38552