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/components/UITheme.cpp

@@ -13,10 +13,6 @@
 #include "components/themes/lyra/LyraTheme.h"
 #include "components/themes/roundedraff/RoundedRaffTheme.h"
 
-namespace {
-constexpr int SKIP_PAGE_MS = 700;
-}  // namespace
-
 UITheme UITheme::instance;
 
 UITheme::UITheme() {