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/app/src/main/java/org/dolphinemu/dolphinemu/model/GameFile.java

@@ -30,6 +30,8 @@ public class GameFile
 
   public native String getGameId();
 
+  public native String getGameTdbId();
+
   public native int getDiscNumber();
 
   public native int getRevision();
@@ -43,7 +45,7 @@ public class GameFile
   public String getCoverPath()
   {
     return Environment.getExternalStorageDirectory().getPath() +
-            "/dolphin-emu/Cache/GameCovers/" + getGameId() + ".png";
+            "/dolphin-emu/Cache/GameCovers/" + getGameTdbId() + ".png";
   }
 
   public String getCustomCoverPath()

Source/Android/app/src/main/java/org/dolphinemu/dolphinemu/utils/CoverHelper.java

@@ -12,10 +12,7 @@ public final class CoverHelper
 
   public static String buildGameTDBUrl(GameFile game, String region)
   {
-    String gameId = game.getGameId();
-    if (game.getPlatform() == 2) // WiiWare
-      gameId = gameId.substring(0, 4);
-    return String.format(baseUrl, region, gameId);
+    return String.format(baseUrl, region, game.getGameTdbId());
   }
 
   public static String getRegion(GameFile game)
@@ -77,4 +74,4 @@ public final class CoverHelper
       // Do nothing
     }
   }
-}
+}