Cache and read feature flags on the disk

[marandaneto]

💡 Motivation and Context

Closes #39

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[benjackwhite]

All looks good to me!

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

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Please add an entry to CHANGELOG.md to the "Next" section. Make sure the entry includes this PR's number.
Example:

## Next
- Cache and read feature flags on the disk ([#61](https://github.com/PostHog/posthog-android/pull/61))

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