Add keepsBluetoothAlive hook to Activity base class

Introduces a virtual method allowing activities to indicate whether they need the BLE stack to remain active. This enables selective teardown of Bluetooth to free heap memory, with the Bluetooth settings screen being able to override this for pairing and scanning functionality.

src/activities/reader/EpubReaderActivity.h

@@ -86,6 +86,7 @@ class EpubReaderActivity final : public Activity {
   void loop() override;
   void render(RenderLock&& lock) override;
   bool isReaderActivity() const override { return true; }
+  void requestGhostCleanup() override { pagesUntilFullRefresh = 1; }
   ScreenshotInfo getScreenshotInfo() const override;
   CrossPointPosition getCurrentPosition() const;
 };