Support overriding asset path and ICU path on Linux #117103
Description
FlDartProject doesn't support overriding the asset directory or the ICU data path, but instead hardcodes them to data/flutter_assets and data/icudtl.dat respectively.
Add support for overriding the defaults. The asset path override is useful for authoring fixture tests. See #117102. The ICU data path is for parity with other desktop embedders which support overriding this value.