[Sb refactor] Add a demo command flow method

This CL mainly just adds a single method to DemoModeController,
`demoFlowForCommand(String)`. The rest of the diff is from a combination
of ktlint formatting and writing a test for the flow, which required
some restructuring of the class.

Test: DemoModeControllerTest
Bug: 238425913
Change-Id: I40dd0b42a881d0776e9b19b282496e850a67a981