feat: implement the analytics SDK as a singleton

[1abhishekpandey]

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.

About the change

• 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.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• 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

[ChryssaAliferi]

Nice effort, requesting some changes for alignment and for better kotlin implementation.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[ChryssaAliferi]

Simplified singleton instance and moving some ext functions to better suited place.
Great work!! Looks good to me!