- Host the WebView and all of its related code inside of a Fragment
instead of an Activity. Fragments can give us more flexibility in the
future on how we can display the WebView, how we could manage some
bottom menu via a BottomNavigationView or similar, etc.

- Use a ViewModel and LiveData architecture in order to move away from
Loaders, which are now deprecated. Almost all state information for the
viewer (like current page, zoom ratio) is now stored in the ViewModel,
which survives configuration changes.
  - Rewrite the document property parsing in DocumentPropertiesLoader in
    Kotlin, taking advantage of Kotlin coroutines to do asynchronous
    parsing of the document properties.

- Dynamically update the properties dialog after PDF loads.
If viewing the PDF properties while the PDF loads, before, the dialog
would show an error message. Now, the error message will be swapped out
with the parsed info as soon the document properties have been parsed.
This is done using an Observer on LiveData.

- Use an alpha version of AndroidX Fragment so that we can use a simpler
way to pass data between two Fragments:
https://developer.android.com/training/basics/fragments/pass-data-between

- Alpha version of Fragments also has ActivityResultLauncher, which
simplifies the SAF launch
https://developer.android.com/training/basics/intents/result
"While the underlying startActivityForResult() and onActivityResult()
APIs are available on the Activity class on all API levels, it is
strongly recommended to use the Activity Result APIs introduced in
AndroidX Activity 1.2.0-alpha02 and Fragment 1.3.0-alpha02."

Closes GrapheneOS#69, closes GrapheneOS#70

- Save state using the ViewModel Saved State module, since the ViewModel
alone doesn't survive system-initiated process death. The ViewModel is
good for configuration changes.
https://developer.android.com/topic/libraries/architecture/viewmodel-savedstate

- Save the document properties state to prevent reparsing and to show
the properties if the Activity gets killed while the properties dialog
is open.

- Don't load null URIs from open document action.

Now, we just reconstruct the properties if the app goes through some
system-initiated process death. For configuration changes, the app
still retains the properties in the ViewModel.

Also,
- Move the WebView into a container so that it can be cleared without
giving errors.
- Refactor document parsing even more (+ move into separate file) and
  conform to Kotlin style guide

At times, the WebView would execute methods via the Channel even when
the Fragment was in a destroyed state. This crashed the app, since the
Fragment wasn't tied to an Activity. This fixes that by explicitly
stopping the WebView when the Fragment's view is destroyed.

Fixes GrapheneOS#71

This means we won't be able to use ActivityResultLauncher and
FragmentResult owners and listeners. This can be reverted later when
it's more stable.

This reverts commit 6260bcb