Rebind MessageHeaderView if on-screen.

Fixes b/10413376. Previously, if the MessageHeaderView
was on screen while its snap header was tapped (to collapse it),
it would not be updated with its new state (since they were two
separate views). The rebind takes care of this.

Change-Id: I5a24338b52e1d0f220a02f8e1ebc92b53814d16c
3 files changed