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/reader/EpubReaderActivity.cpp

@@ -320,6 +320,7 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
           std::make_unique<EpubReaderChapterSelectionActivity>(renderer, mappedInput, epub, path, spineIdx),
           [this](const ActivityResult& result) {
             if (!result.isCancelled && currentSpineIndex != std::get<ChapterResult>(result.data).spineIndex) {
+              RenderLock lock(*this);
               currentSpineIndex = std::get<ChapterResult>(result.data).spineIndex;
               nextPageNumber = 0;
               section.reset();
@@ -421,6 +422,7 @@ void EpubReaderActivity::onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction
               if (!result.isCancelled) {
                 const auto& sync = std::get<SyncResult>(result.data);
                 if (currentSpineIndex != sync.spineIndex || (section && section->currentPage != sync.page)) {
+                  RenderLock lock(*this);
                   currentSpineIndex = sync.spineIndex;
                   nextPageNumber = sync.page;
                   section.reset();