This adds infrastructure to read/write preferences, with both a multiplatform wrapper and an Android-specific implementation.
This implementation is primarily designed to cover the initial needs of implementing the wallet SDK integration for issue #28 for securely storing keys (with encryption) for the user's wallet.
This implements the logical flow between the onboarding stages, as well as nearly full test coverage of the flow.
There's a lot of followup work, especially to implement design styles. It currently contains various placeholder values, e.g. lorem ipsum text and color blocks instead of images. See #17