Track if the app is terminated by the OS to free memory

## What feature would you like to see?

When our app is terminated by the OS to free memory, we need to know about it.

## How would you use it?

For us, we have a flutter app that is occasionally terminated when in the background to free up memory. It's been very difficult to diagnose as it never happens during testing (only customer reports to go on). A lack of data in crashlytics has made the issue invisible to us.

https://github.com/flutter/flutter/issues/48016#issuecomment-971918009