Support changing FAB icon to add person.

+ Add function to change the icon on the FAB. Should likely
eventually include an optional animate parameter.
+ Change FAB controller so that it recognizes the FAB button
as an ImageButton.
+ Flip add person icons to white.

Bug: 19366434
Change-Id: I5f6cf07ffce1d0eaa1aa0733f7411247cba1d6c0