[jni] Remove hack in `findClass`

[liamappelbe]

As part of the extension type migration, I had to add this hack to findClass:

static JClassPtr findClass(String name) {
  // HACK:
  if (name.startsWith('L') && name.endsWith(';')) {
    name = name.substring(1, name.length - 1);
  }
  return using((arena) => _bindings.JniFindClass(name.toNativeChars(arena)))
      .checkedClassRef;
}

We should find all the places where the signature is being used for class lookup, and switch them over to pass only the class name.