Use duck typing in X509TrustManagerExtensions

X509TrustManagerExtensions assumes that the default X509TrustManager is
an instance of conscrypt's TrustManagerImpl. That's no longer going to
always be the case. Instead use duck typing to support any
X509TrustManagers that have the extra methods required for
X509TrustManagerExtensions.

Change-Id: If23471bda590d5e131bb1e802a60599957bc7f37