Fix distance display preferring stale route distance over manually updated quantity

[neil-marcellini]

Explanation of Change

In getDistanceInMeters, the route distance (transaction.routes.route0.distance) was checked first and took priority over comment.customUnit.quantity. When a user manually updates the distance value in Expensify Classic, UpdateTransaction writes only to comment.customUnit.quantity but does not clear routes.route0.distance. This caused the stale route value to be displayed instead of the manually updated distance.

The fix reverses the priority: prefer customUnit.quantity when it is set (since it reflects the user's manual update), and only fall back to routes.route0.distance when no custom quantity exists (e.g. during initial distance request creation before the route is completed).

Fixed Issues

$ #86568

Tests

1. Create an expense on New Expensify using Track distance and the map
2. Enter any two waypoints and save the expense
3. Open the same expense on Expensify Classic
4. Manually update the distance value and save the expense
5. Reopen the expense on New Expensify
6. Verify the distance field shows the manually updated value, not the original route distance

• Verify that no errors appear in the JS console

Offline tests

N/A - This is a display logic fix that does not involve network requests.

QA Steps

1. Create an expense on New Expensify using Track distance and the map
2. Enter any two waypoints and save the expense
3. Open the same expense on Expensify Classic
4. Manually update the distance value and save the expense
5. Reopen the expense on New Expensify
6. Verify the distance field shows the manually updated value, not the original route distance

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
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  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
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  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari