fix: ActivityManager tweaks (#1220)

## Summary

**What is the goal of this PR?**

Small tweaks to #1016:
- Only Activity and ActivityManager can access activityResultHandler and
activityResult
- `[[maybe_unused]]` in RenderLock constructor
- Only ActivityManager and RenderLock can access renderingMutex
- Missing renderUpdate after failed wifi selection
- Standardize on activities calling finish instead of
activityManager.popActivity
- Hold RenderLock while mutating state in EpubReaderActivity result
handlers

---

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src/activities/ActivityManager.h

@@ -11,7 +11,6 @@
 
 #include "GfxRenderer.h"
 #include "MappedInputManager.h"
-#include "RenderLock.h"
 
 class Activity;    // forward declaration
 class RenderLock;  // forward declaration
@@ -31,6 +30,8 @@ class RenderLock;  // forward declaration
  * - onActivityResult is implemented via a callback instead of a separate method, for simplicity
  */
 class ActivityManager {
+  friend class RenderLock;
+
  protected:
   GfxRenderer& renderer;
   MappedInputManager& mappedInput;
@@ -49,6 +50,10 @@ class ActivityManager {
   static void renderTaskTrampoline(void* param);
   [[noreturn]] virtual void renderTaskLoop();
 
+  // Mutex to protect rendering operations from race conditions
+  // Must only be used via RenderLock
+  SemaphoreHandle_t renderingMutex = nullptr;
+
   // Whether to trigger a render after the current loop()
   // This variable must only be set by the main loop, to avoid race conditions
   bool requestedUpdate = false;
@@ -61,10 +66,6 @@ class ActivityManager {
   }
   ~ActivityManager() { assert(false); /* should never be called */ };
 
-  // Mutex to protect rendering operations from race conditions
-  // Must only be used via RenderLock
-  SemaphoreHandle_t renderingMutex = nullptr;
-
   void begin();
   void loop();