Remove container class for intro slides

Even with passing a bundle with the layout ID sometimes
some users will encounter a situation where the _layout
variable is null. To avoid further issues, creating a
class for each intro slide, so the layout is encoded
into the class itself.

app/src/test/java/protect/card_locker/MainActivityTest.java

@@ -131,7 +131,7 @@ public class MainActivityTest
 
         ComponentName componentName = next.getComponent();
         String name = componentName.flattenToShortString();
-        assertEquals("protect.card_locker/.IntroActivity", name);
+        assertEquals("protect.card_locker/.intro.IntroActivity", name);
 
         Bundle extras = next.getExtras();
         assertNull(extras);