Make projectID and preview API configurable

[Simply007]

Motivation

Internal link: DEVREL-694

Make the project Id and preview API key configurable. Currently using getIntent.getStringExtra().

• In combination with appetize it can be configurable => check i.e. https://github.com/kontent-ai/safelife-appetize-iframe

Checklist

• [ ] Code follows coding conventions held in this repo
• [ ] Automated tests have been added
• [ ] Tests are passing
• [ ] Docs have been updated (if applicable)
• [ ] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[IvanKiral]

Requested changes:

• [x] Add how to use customProjectId to readme
• [ ] Consider if current approach of customProjectId is right
• [x] Considre renaming resetClient

[Simply007]

Requested changes:

• [x] Add how to use customProjectId to readme
• [ ] Consider if current approach of customProjectId is right

I think the code is fine - it is a sample..

• [x] Considre renaming resetClient