Add demo-ui-lib module

[HonzaR]

Is your feature request related to a problem? Please describe.

This is a Benchmarking Demo-app task https://github.com/zcash/zcash-android-wallet-sdk/pull/805 follow up. The aim is to refactor the Demo-app to have demo-ui-lib so that we can access the string resources programmatically.

Describe the solution you'd like

We might have to tweak our build scripts so that they don’t publish the ui-lib to maven central. Also, don't forget to update the architecture/setup documentation after adding this new module.

Additional context

It would be also great to reference texts from the Demo-app to the Benchmarking module tests to remove the hardcoded strings there as part of this work.