Late binding: support children of Signature

The way Signature is a class of SignatureSpi allows you to register a
provider that returns a direct child of Signature. Support that by
checking if it's a subclass before trying to wrap it in SignatureImpl.

Bug: 12971079
Change-Id: I76b03f0962fe1b6ae95f205588c17d54b4bf504c
1 file changed