fix: update CustomBranchApp to use getInstance(Context) method

- Replace Branch.getInstance() with Branch.getInstance(this) in CustomBranchApp
- Provide proper context for Branch SDK initialization
- Store branch instance in local variable to avoid multiple calls
- Fixes NullPointerException crash during application startup
- Ensures proper Branch SDK initialization in TestBed application

Author: wpinho-branch

Branch-SDK-TestBed/src/main/java/io/branch/branchandroidtestbed/CustomBranchApp.java

@@ -26,11 +26,11 @@ public void onCreate() {
 //            saveLogToFile(message);
 //        };
         Branch.enableLogging(BranchLogger.BranchLogLevel.VERBOSE);
-        Branch.getInstance();
+        Branch branch = Branch.getInstance(this);
         CustomTabsIntent customTabsIntent = new CustomTabsIntent.Builder()
                 .setColorScheme(COLOR_SCHEME_DARK)
                 .build();
-        Branch.getInstance().setCustomTabsIntent(customTabsIntent);
+        branch.setCustomTabsIntent(customTabsIntent);
     }
 
     private void saveLogToFile(String logMessage) {