Refactoring Part 1: getString() as a member

Replace instances of messy function calls with cleaner member variable
names.
Revert "Refactoring Part 1: getString() as a member"

This reverts commit 374de89df9ea429f61954f1d229769f15cb58fa2.
Refactoring 2: Move SendMessageResultCallback out

ResultCallback code isn't doing anything functionally important inside
sendMessage so move it out
Refactoring 3: Debug Messages

Print the JSON'd message when it successfully / unsuccessfuly is sent,
don't toast the message if you're not on Chromecast because it's
awkwardly long maybe.
Refactoring 4: Split Up sendMessage into 3

We now have a separate method for each of sendMessage's stages:
1. Pack the button + text into JSON
2. Convert the JSON to a string
3. Send the string to the server