* Closes#2770: Allow receiving tabs from FxA devices
Now that we're on a-c 0.54, we can land this since it supports device
capability migration.
This patch adds a SEND_TAB device capability, making Fenix a valid target
in the Send Tab device list on Desktop Firefox.
Additionally, it adds a notification manager which manages notification
channels and knows how to display "received tabs" notifications".
* Post: remove unusued test file that's causing issues
Jeff Boek