Convert ReaderPostPagerActivity to Kotlin (#21822)

* Make ReaderPostPagerActivity fullscreen

* Reverted custom theme

* Use EdgeToEdge.enable

* Converted pager activity to ConstraintLayout

* Don't overlay navigation bar

* Hide navigation bar

* Only enable fullscreen for Android 33+

* Use non-deprecated fullscreen

* Rename .java to .kt

* First pass at converting ReaderPostPagerActivity to Kotlin

* Second pass at converting ReaderPostPagerActivity to Kotlin

* Use lateinit var

* Fixed unnecessary nonnull assertions

* Suppress deprecation warnings

* First pass at renaming variables

* Second pass at renaming variables

* Use BundleCompat for serializable extras

* Suppress unchecked cast

* Renamed adapter variables

* Moved adapter

* Mark class deprecated

* Removed Java activity

* Fixed Detekt warnings

* Use Kotlin isNullOrEmpty instead of TextUtils.isEmpty

Author: nbradbury