Align documentation and code sample on how to manually track screens about snippets-android

[thatfiredev]

Done in #408

Thanks @AlbertoYabeda

[thatfiredev]

@flyrev you're right, the screen_view event should be logged from onResume. This repo (snippets-android) is only used as a backing source for our docs, so most of the times we don't really take in consideration where each method is being called from. Our quickstart samples are the ones that we tell developers to use as "code samples", and it already calls it in onResume:

https://github.com/firebase/quickstart-android/blob/6945fb0a9d20b5dc3597209493e30792022b08f7/analytics/app/src/main/java/com/google/firebase/quickstart/analytics/kotlin/MainActivity.kt#L99

For consistency, however, I would be happy to change the call in this repo from onCreate to onResume. Let me know if you'd like to send us a PR :)

[albertoazinar]

Hello, Can you assign it to me? I want to try

[albertoazinar]

Always @thatfiredev