Test for null context in getPrivateKey

In N, passing a null context to getPrivateKey provoked a
NullPointerException, which is validated by a CTS test. In commit
28d68b14 this behavior was changed (inadvertently, I believe) causing
getPrivateKey to wrap the NPE in a KeyChainException. This CL restores
the previous behavior, fixing the test and avoiding breaking any apps
that were catching the NPE.

Test: Fixing broken test
Change-Id: Icb0c75b03efc478b7310998cf3e7108a2c419107