refactor: Simplify isReaderActivity bookkeeping (#1838)

## Summary

Before, any sub-activity of a reader activity needed to override
`isReaderActivity` to maintain correct bookkeeping through
`ActivityManager::isReaderActivity`. We could easily miss this in any
new sub-activities. Instead, simplify so each reader activity correctly
reports and then `ActivityManager` checks for any reader activity in its
stack.

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src/activities/reader/EpubReaderMenuActivity.h

@@ -32,7 +32,6 @@ class EpubReaderMenuActivity final : public Activity {
   void onExit() override;
   void loop() override;
   void render(RenderLock&&) override;
-  bool isReaderActivity() const override { return true; }
 
  private:
   struct MenuItem {