Add notification system

[joe-trellick]

Right now, Cicada is good for making glanceable displays, but it doesn't deal with notifications of events happening in the Android system at all. We should add a mechanism for that.

[joe-trellick]

I've made some early progress on this in this change:
edc49fc

and the one before it. Other apps can now send text-based or (untested) bitmap-based notifications to Cicada. You should definitely expect the API to change and get refined over time.