Feature: Resume video player where user left it

[astarivi]

Current player code makes it impossible to skip to a part of the video that hasn't been downloaded yet.

Introduce code to allow this to happen, then introduce resume at a certain timestamp. These timestamps are already saved in the Room database.

To achieve this behaviour, the user could relinquish control of the UI after seeking, and run a task in the background to check if the video can be played yet. Once the video can be played, give control back to the user. (Remember to allow user to seek, but not pause/resume or skip)

Relevant code: https://github.com/astarivi/Kaizoyu/tree/main/app/src/main/java/com/astarivi/kaizoyu/video