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.cpp

@@ -232,7 +232,7 @@ RenderLock::RenderLock() {
   isLocked = true;
 }
 
-RenderLock::RenderLock(Activity& /* unused */) {
+RenderLock::RenderLock([[maybe_unused]] Activity&) {
   xSemaphoreTake(activityManager.renderingMutex, portMAX_DELAY);
   isLocked = true;
 }