KeyManager.choose* methods should tolerate null key types

This regression was found by X509KeyManagerTest and now
KeyManagerFactoryTest covers it as well. The underlying problem was
introduced recently when KeyManagerImpl was updated to support key
types with specific signature algorithms like EC_RSA and EC_EC.

Change-Id: Ic99ab10e5ba07e990dc0e8a2d257c2167f2d33bb