chore: Add FIXME comments for unused and potentially buggy code

This commit adds `FIXME` comments to several parts of the codebase to highlight areas that require attention.

Specifically, it marks:
*   Unused methods in `MainViewModel`, `SettingsViewModel`, `SupportViewModel`, and `ReviewHelper`.
*   A potential null argument being passed in `DefaultSupportRepository`.

Author: Mihai-Cristian Condrea

app/src/main/java/com/d4rk/androidtutorials/java/data/repository/DefaultSupportRepository.java

@@ -131,7 +131,7 @@ public BillingFlowLauncher initiatePurchase(String productId) {
                     Collections.singletonList(
                             BillingFlowParams.ProductDetailsParams.newBuilder()
                                     .setProductDetails(details)
-                                    .setOfferToken(finalOfferToken)
+                                    .setOfferToken(finalOfferToken) // FIXME: Argument 'finalOfferToken' might be null
                                     .build()
                     );
 

app/src/main/java/com/d4rk/androidtutorials/java/ui/screens/main/MainViewModel.java

@@ -99,14 +99,14 @@ public void applySettings(String[] themeValues,
     /**
      * Checks if we need to show the startup screen.
      */
-    public boolean shouldShowStartupScreen() {
+    public boolean shouldShowStartupScreen() { // FIXME: Method 'shouldShowStartupScreen()' is never used
         return shouldShowStartupScreenUseCase.invoke();
     }
 
     /**
      * Mark startup screen as shown.
      */
-    public void markStartupScreenShown() {
+    public void markStartupScreenShown() { // FIXME: Method 'markStartupScreenShown()' is never used
         markStartupScreenShownUseCase.invoke();
     }
 

app/src/main/java/com/d4rk/androidtutorials/java/ui/screens/settings/SettingsViewModel.java

@@ -70,7 +70,7 @@ public String getDarkMode() {
         return getDarkModeUseCase.invoke();
     }
 
-    public void setConsentAccepted(boolean accepted) {
+    public void setConsentAccepted(boolean accepted) { // FIXME: Method 'setConsentAccepted(boolean)' is never used
         setConsentAcceptedUseCase.invoke(accepted);
     }
 }

app/src/main/java/com/d4rk/androidtutorials/java/ui/screens/support/SupportViewModel.java

@@ -47,7 +47,7 @@ public SupportRepository.BillingFlowLauncher initiatePurchase(String productId)
         return initiatePurchaseUseCase.invoke(productId);
     }
 
-    public AdRequest initMobileAds() {
+    public AdRequest initMobileAds() { // FIXME: Method 'initMobileAds()' is never used
         return initMobileAdsUseCase.invoke();
     }
 }

app/src/main/java/com/d4rk/androidtutorials/java/utils/ReviewHelper.java

@@ -25,7 +25,7 @@ public static void launchInAppReviewIfEligible(Activity activity,
         launchReview(activity, onReviewLaunched);
     }
 
-    public static void forceLaunchInAppReview(Activity activity) {
+    public static void forceLaunchInAppReview(Activity activity) { // FIXME: Method 'forceLaunchInAppReview(android.app.Activity)' is never used
         launchReview(activity, null);
     }