fix: import com.segment.analytics.Properties in generated Android clients [SCH-1670]

[colinking]

Our Android example application stores Typewriter generated files at com.segment.analytics, which concealed a bug in #57 where com.segment.analytics.Properties (from analytics-android) was not being imported.

This PR:

• Moves the generated Typewriter client in the Android example app to com.segment.generated.
• Adds com.segment.analytics.Properties as an import in generated files.
• Switches List<Object> for List<Properties, since Objects would otherwise just be .toString-ed when the event is fired
• Fixes serialization for List<T> by casting to List<Properties>

[tyronen]

Hi I was looking into using gen-android in a project, but this bug is a blocker - without this PR, the generated code does not compile.

The docs also don't mention that the --trackingPlan argument is required.

[colinking]

Thanks for the +1 @tyronen! We're personally focused on shipping Typewriter v7 (#59) which'll include a fix for this, along with a set of regression tests for this kind of issue.

We don't yet have an Android client ready in that pre-release (it'll be available soon), but always open to any feedback you have on the current Android client!