add isIdentified property and check on identify()

[beradeep]

:bulb: Motivation and Context

This change adds the is_identified property (absent earlier) to check user is identified or not (anonymous). This change also adds a fix to identify() to ensure that an "identify" event is captured if and only if the user isn't already identified (anonymous). Also adds an in-memory flag to check if the property is already loaded or not.

fix https://github.com/PostHog/posthog-android/issues/136 & https://github.com/PostHog/posthog-android/issues/108

:green_heart: How did you test it?

• Added unit test to assert against duplicate "identify" captures
• Added unit test to assert "identify" works as usual on reset
• Verified that this change doesn't break existing tests

:pencil: Checklist

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

[marandaneto]

Thanks @beradeep I left a few comments, after addressing, we can merge it don't forget to add an entry to the changelog.

[marandaneto]

The flag $is_identified isn't being set though, so this issue can't be closed https://github.com/PostHog/posthog-android/issues/136

[marandaneto]

The flag $is_identified isn't being set though, so this issue can't be closed #136

i pushed 2 commits to fix this