routing auto instrumentation should have `navigation` operation instead of `ui.load`

[marandaneto]

Related to #643
Currently, we're using ui.load but it should be navigation following the docs https://develop.sentry.dev/sdk/performance/span-operations/
React native also uses navigation for the routing instrumentation.
https://github.com/getsentry/sentry-react-native/blob/main/src/js/tracing/reactnavigationv4.ts#L288-L298