feat: implement the analytics SDK as a singleton

About the new feature

We have decided to implement the singleton pattern for the analytics SDK.
Multiple instance support is still present in the SDK in case we decide to support multiple instances in the future version of the SDK.
Now the user will always get an analytics instance when calling getInstance method. We have made sure that getInstance doesn't return null.

Changed createInstance API to getInstance and made it return a singleton instance. This logic has been moved into the new RudderAnalytics class.
Removed getInstance method which can return null.
Multiple instance support is still intact. We can use it in future when needed.
Removed the multiple instance-related code from the sample app.
Updated the test case to reflect the singleton pattern.
Moved AnalyticsUtil into the utilities package.

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update
Checklist:
[ ] Version upgraded (project, README, gradle, podspec etc)
[ ] My code follows the style guidelines of this project
[ ] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have added unit tests for the code
[ ] I have made corresponding changes to the documentation