Switching to Volley for HTTP ops & caching.

Volley, as in:
https://android.googlesource.com/platform/frameworks/volley.git

This CL:
- Drops most direct HTTP connections and switches to Volley
- Adds some use-case-specific subclasses of Volley objects
- Includes some refactoring for tidiness
- Adds a crude code-path to actually load images into EventDetailsFragment

This should be most of the infrastructure needed to actually start efficiently
fetching and displaying security camera images in the app UI.