refactor: Simplify XtcReaderActivity with detectPageTurn (#1837)

## Summary

Simplify duplicated code in `XtcReaderActivity` to use
`ReaderUtils::detectPageTurn`. This implementation is now shared with
`EpubReaderActivity` and `TxtReaderActivity`. Also deduplicated the
chapter skip time constant.

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src/activities/reader/TxtReaderActivity.cpp

@@ -71,7 +71,7 @@ void TxtReaderActivity::loop() {
     return;
   }
 
-  auto [prevTriggered, nextTriggered, fromTilt] = ReaderUtils::detectPageTurn(mappedInput);
+  const auto [prevTriggered, nextTriggered, fromTilt] = ReaderUtils::detectPageTurn(mappedInput);
   if (!prevTriggered && !nextTriggered) {
     return;
   }