Add Context.createContextAsUser()

Without it, apps (mainline modules) will need to use createPackageContext...,
which is a bit painful.

Bug: 142472686
Test: atest android.content.cts.ContextTest#testCreateContextAsUser
Change-Id: Id640e03862462724df1a4a3101f0b08faafba22f

test-mock/src/android/test/mock/MockContext.java

@@ -756,6 +756,12 @@ public class MockContext extends Context {
         throw new UnsupportedOperationException();
     }
 
+    /** {@hide} */
+    @Override
+    public Context createContextAsUser(UserHandle user) {
+        throw new UnsupportedOperationException();
+    }
+
     /** {@hide} */
     @Override
     public int getUserId() {