add an object to control screen navigation & add a split-bill dummy screen to use it

This MR moves the test UI stuff out to the "dashboard" and renames dashboard "test". That whole screen should go away eventually. It makes the home screen look a little bit more like Wally's with send and split bill buttons, and an "Accounts" header.

It then creates a class to remember where the user is in the UI "screen" implementing back semantics via a stack, and up semantics via a function that returns what screen is up from the current one. It changes the bottom nav and Android back button to use this class.