SSLSocketTest: TLS client auth with opaque keys

AndroidKeyStore can be used with TLS client auth, but we don't test
anything similar with SSLSocketTest. Add a PrivateKey wrapper that
allows us to closely simulate the conditions which trigger the special
code in Conscrypt to do upcalls to Java to generate signatures with the
client private key.

Test: cts-tradefed run cts -m CtsLibcoreTestCases -a arm64-v8a
Test: cts-tradefed run cts -m CtsLibcoreOkHttpTestCases -a arm64-v8a
Bug: 31714503
Change-Id: I559db546ddd31f8efbe73fc70a91689ed6d7d7e5