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
28d68b1 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

keystore/java/android/security/KeyChain.java

@@ -414,6 +414,9 @@ public final class KeyChain {
         if (alias == null) {
             throw new NullPointerException("alias == null");
         }
+        if (context == null) {
+            throw new NullPointerException("context == null");
+        }
 
         final String keyId;
         try (KeyChainConnection keyChainConnection = bind(context.getApplicationContext())) {