refactor(examples/todos): update flutter code

This is an initial pass to get the todo example into a condition that allows
multiple contributors to work on it.

* Updates flutter code to match version: cc93170894
* Adds rough screens based on the Vanadium Todos example

Remaing tasks are being tracked in vanadium/baku#1 and will follow as separate

Change-Id: I2b1fb33fb5addfbdb844ae0163c56b724ed72ffa
47 files changed