Modernize code base and enable saving to external storage

[jaredning]

Basically, I forked the repo, modernized the code and made some of the changes I've been wanting to see for a while and it works great for my purposes. I doubt anyone else is still using this (although I'll note I have yet to find a better alternative that works on my Android 7.1.1 tablet) but just in case - here's the PR:

• Migrate app to AndroidX and modernize and migrate all dependencies to current versions so that it can successfully compile again
• Enable external storage access so that users can store chapters on SD cards, instead of being locked to internal storage
• Fix Browse > Series tab not displaying any results at all, and add pagination buttons to easily scroll through all series listed on DynastyScans
• Add chapter download button when viewing a series so that you can queue up chapters for download without having to click into each chapter
• Optimize performance around reading/writing chapters to/from external storage
• Optimize loading chapters from external storage for reading to match the speed of the original app when reading off internal storage as closely as possible
• Other performance optimizations to improve performance on older devices, including:
  • Switch to 16 bit rendering with RGB_565
  • Flatten Reader view hierarchy
  • Optimize render loop and scrolling
  • Optimizations when reading from external storage via SAF to speed this up

And yes, this was 100% done with Gemini - The file system changes I probably could've figured out on my own, but I have zero Android experience so tackling a migration of this size would've been impossible otherwise.

The original functionality should've all been more-or-less maintained, as far as I can tell - although I worry some of the changes around keeping the delete chapter functionality working are probably pretty close to AI slop, but my goal was just to make the improvements I needed without breaking the rest of the app.

Also, this breaks compatibility with Android versions pre 6.0 - seems worth noting considering the original app in its current form supports Android 4.1.

I created my own APK from my fork here for download if anyone's interested.

EDIT: I'm still fixing bugs as I find them, but this is close to being at the very least stable, with functionality akin to the original app. There are some QOL improvements I can think of that would be worth doing, especially to streamline things for users other than me, such as:

• Multiple source folders (i.e. let the app download and read from both internal and external storage at the same time)
• Allow importing from pre-existing source folders to sync DB state with chapters/series that have already been downloaded

I'll see how I feel about these based on my testing / personal use - but for now, the goal now is to ensure the app is stable, and most importantly that the original functionality is also preserved on top of the changes I've added.