[java] Appropriate instantiation of model with dynamic properties

This is an intermediary work around for issues such as #3456. This fixes #5920 (closed).

There's still an issue with applying the import of HashMap when referencing a map-like type. For an example of the issue, generate with the new mapSchemas.yaml test file and inspect the FreeformObjectTypes type.

Applying container import logic similar to what's done in DefaultCodegen:

// TODO revise the logic to include map
if (cp.isContainer) {
    addImport(m, typeMapping.get("array"));
}

for map does not work as expected because Java defines the type mapping as java.util.Map and the instantiation type as java.util.HashMap. It seems like the above snippet in DefaultCodegen might need to account for both the type mapping and the instantiation type when modifying a model's imports.

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