If the mapping callback is not set or it the callback returns invalid data, ICU initialization will be embedder responsibility.
This affects all embedders and the following have been audited:
* Android: Via a symbol mapping.
* iOS: Via a file mapping.
* Embedder: Via a file mapping.
* Fuchsia: Via a VMO mapping
* Test shells and Flutter tester: Via file mapping with ICU data needing to be next to the executable.
Prior to this the Android embedder code would extract the icudtl.dat asset out
of the APK and write it to local disk during the first startup of the app.
This change will make that work unnecessary and eliminate the risk of ICU
failures due to errors in the extraction process.
