unified_analytics should export `DashEvent` and `DevicePlatform`

[goderbauer]

See flutter/flutter#143594. Consumers currently have to use these via src imports, which the implementation_imports flags.

Justification for exporting them:

DashEvent is referenced in the public API of the package (example: https://pub.dev/documentation/unified_analytics/latest/unified_analytics/Event/eventName.html), therefore it should be exported and be publicly available as well.

DevicePlatform is expected to be used in tests.

[goderbauer]

@eliasyishak FYI