[Performance] Add Sentry spans for Camera FAB → Scan page flow

[cristipaval]

Coming from #83502.

We need proper Sentry instrumentation for the Camera FAB → Scan page transition. Specifically, a parent span measuring the full time from FAB tap to camera ready, with two subspans:

1. Navigation to the Scan page
2. Scan page render until the camera is ready

This will help us track the impact of the perf improvements we've already shipped and catch regressions going forward.

Issue Owner

Current Issue Owner: @rinej

[rinej]

I created the PR -> #87765