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/ReaderUtils.h

@@ -10,6 +10,7 @@
 namespace ReaderUtils {
 
 constexpr unsigned long GO_HOME_MS = 1000;
+constexpr unsigned long SKIP_HOLD_MS = 700;
 
 inline void applyOrientation(GfxRenderer& renderer, const uint8_t orientation) {
   switch (orientation) {