Use smallest width instead of width for alt layouts

(cherry picked from commit 04ebca8413403693870e58540eda02b1391f3b61)

# Conflicts:
#	app/src/main/res/layout-sw720dp/manga_info_header.xml

app/src/main/java/eu/kanade/tachiyomi/util/system/ContextExtensions.kt

@@ -271,8 +271,8 @@ fun Context.createFileInCacheDir(name: String): File {
 }
 
 /**
- * We consider anything with a width of >= 720dp as a tablet, i.e. with layouts in layout-w720dp.
+ * We consider anything with a width of >= 720dp as a tablet, i.e. with layouts in layout-sw720dp.
  */
 fun Context.isTablet(): Boolean {
-    return (resources.displayMetrics.widthPixels / resources.displayMetrics.density) >= 720
+    return resources.configuration.smallestScreenWidthDp >= 720
 }