refactor(react-native): Ensure session is not started in the JS layer
refactor(react-native): Ensure session is not started in the JS layer

If session tracking is enabled by default, a session will already have been started in the native
layer and this would result in duplicate startSession() call.

bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js