Add Platform backup to Amplitude Actions for parity with Amplitude Classic (Cloud Mode)

[Innovative-GauravKochar]

https://segment.atlassian.net/browse/HGI-166

This Pr is to make So that platform should by default maps to context.library.name and falls back to context.device.type.

Testing

Include any additional information about the testing you have completed to ensure your changes behave as expected. For a speedy review, please check any of the tasks you completed below during your testing.

• [x] Added unit tests for new functionality
• [x] Tested end-to-end using the local server
• [x] [Segmenters] Tested in the staging environment

[Innovative-GauravKochar]

Amplitude (Actions) platform by default maps to context.library.name and falls back to context.device.type for LogEvent,LogEventV2,LogPurchase,IdentifyUser