[jnigen] JNumber subclasses cannot be generated

[HosseinYousefi]

We could use toDartInteger and toDartDouble to match JString instead of adding the releaseOriginal parameter to existing methods like intValue or doubleValue. This also frees intValue() and such for the subclasses. Currently generating java.math.BigInteger fails since package:jni's JNumber has a intValue that does not have a releaseOriginal: true. We could potentially add this recursively to all of the subclasses as well (or simply not generate these methods as we know JNumber always exists.

A mechanism to modify the bindings of the dependent packages via the symbols file is nice to have nonetheless.