Prevent Echo mention notifications for MassMessage messages

Hook into Echo, and abort any mention notifications if the user
is the bot, and the type is 'mention'.

Depends on I376708ac7aafc419bf87bceeff6de7de9bdc2f4e in Echo.

Change-Id: If8ee6acde77e2d0905aa85ca90625fc915784410
Merge "Prevent Echo mention notifications for MassMessage messages"