FR: Add screentrace with custom name and start / stop methods rather than per viewcontroller

### Feature proposal

* Firebase Component: Performance

A React Native "screen" can be anything and doesn't have to be a ViewController. In fact, most everything in React Native runs in one ViewController. Thus I am proposing an API:

ScreenTrace.start('FirstScreen');
ScreenTrace.stop('FirstScreen');

ScreenTrace.start('SecondScreen');
ScreenTrace.stop('SecondScreen');

Allowing users to use this approach might override the current approach of going per-ViewController, so maybe there's also a boolean or switch to move to user controlled ScreentTaces.