Make IdentityCredentialStore.getInstance() return null if credstore is not installed.

Having this method return null is the expected and documented behavior
when either the IC HAL or credstore isn't available.

Test: atest android.security.identity.cts (with credstore not running)
Bug: 148495024
Change-Id: Ifa17c58a84057499b1aeb8404959d5c0badfe52a

identity/java/android/security/identity/CredstoreIdentityCredentialStore.java

@@ -38,6 +38,10 @@ class CredstoreIdentityCredentialStore extends IdentityCredentialStore {
         ICredentialStoreFactory storeFactory =
                 ICredentialStoreFactory.Stub.asInterface(
                     ServiceManager.getService("android.security.identity"));
+        if (storeFactory == null) {
+            // This can happen if credstore is not running or not installed.
+            return null;
+        }
 
         ICredentialStore credStore = null;
         try {