[Odometer] Save odometer readings and image data to device storage

[Julesssss]

Add Onyx-backed draft persistence for odometer expense data.

Design Doc section: https://docs.google.com/document/d/1Z103dY4guA07UUw3sTP1o0IKvFxXZJ1HAB1EwUeEyaA/edit?tab=t.0#bookmark=id.le7upbg1v6r1

Implementation:

• Add ODOMETER_DRAFT: 'odometerDraft' key to ONYXKEYS.ts
• Create IOU.saveOdometerDraft(startReading, endReading, startImage, endImage) in IOU.ts
• Create IOU.clearOdometerDraft() — clears draft after successful expense submission
• Web/mWeb/Desktop: Convert File objects to base64 via FileReader.readAsDataURL(), store as data URIs
• iOS/Android: Store file URI strings directly (images persist on device filesystem)
• Call clearOdometerDraft() in the expense submission flow after successful API response

Reference: Odometer tracking issue — Release 2

Issue Owner

Current Issue Owner: @jakubkalinski0

[melvin-bot]

@jakubkalinski0 Whoops! This issue is 2 days overdue. Let's get this updated quick!

[jakubkalinski0]

This issue is being actively worked on here:

• [Odometer] Add save for later functionality #87726