Keep track of GameTDB ID separately from game ID

The difference between Dolphin's game IDs and GameTDB's game IDs
is that GameTDB uses four characters for non-disc titles, whereas
Dolphin uses six characters for all titles.

This fixes:

- TitleDatabase considering Datel discs to be NHL Hitz 2002
- Gecko code downloading not working for discs with IDs starting with P
- Cover downloading mixing up discs with channels (e.g. Mario Kart Wii
  and Mario Kart Channel) and making extra HTTP requests. (Android was
  actually doing a better job at this than DolphinQt!)

Source/Android/jni/GameList/GameFile.cpp

@@ -123,6 +123,12 @@ JNIEXPORT jstring JNICALL Java_org_dolphinemu_dolphinemu_model_GameFile_getGameI
   return ToJString(env, GetRef(env, obj)->GetGameID());
 }
 
+JNIEXPORT jstring JNICALL Java_org_dolphinemu_dolphinemu_model_GameFile_getGameTdbId(JNIEnv* env,
+                                                                                     jobject obj)
+{
+  return ToJString(env, GetRef(env, obj)->GetGameTDBID());
+}
+
 JNIEXPORT jint JNICALL Java_org_dolphinemu_dolphinemu_model_GameFile_getDiscNumber(JNIEnv* env,
                                                                                    jobject obj)
 {