[browser] Fix custom icu fingerprinting.

[ilonatommy]

Using custom ICU files got broken with fingerprinting.
Repro:

<BlazorIcuDataFileName>/workspaces/runtime/artifacts-samples/browser/icudt_custom.dat</BlazorIcuDataFileName>

produces boot.config.json:

  "mainAssemblyName": "browser",
  "resources": {
    "hash": "sha256-mDtUMfPuXPyYADO8g8Kk3TOZPMhbHv/IMatrIfcVy8M=",
    "fingerprinting": {
         ...........
        "icudt_custom.oh1zvcfom8.dat": "icudt_custom.dat",
     },
    "icu": {
      "icudt_custom.oh1zvcfom8.dat": "sha256-tO5O5YzMTVSaKBboxAqezOQL9ewmupzV2JrB5Rkc8a4="
    },
   }
  "globalizationMode": "custom"

fileMapping has contents: mapping={ "icudt_custom.dat": "icudt_custom.oh1zvcfom8.dat" }
icuFile has a value icudt_custom.oh1zvcfom8.dat.
So we won't enter the condition:

        if (icuFile && fileMapping[icuFile]) {
            return fileMapping[icuFile];
        }

and would finish with invariant mode that prevents globalization methods from working.

ToDo:
should be backported to net9.

[ilonatommy]

/backport to release/9.0-rc2

[github-actions]

Started backporting to release/9.0-rc2: https:/dotnet/runtime/actions/runs/10944660037

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