Added support for disc number (#18)

[Aga-C]

What is it?

• Bugfix
• Feature
• Codebase improvement

Description of the changes in your PR

• Added support for disc number metadata.
  • Reading that metadata from Audio.Media is supported since API 30, but reading from file since API 10. That's why in manual file scan I haven't added an Android version check.
• If there's a disc number, it's displayed in an album view.
• Sorting by track number also considers disc numbering.
• As I've seen, Android keeps track number as a string, but MediaMetadata.Builder requires it to be a number, so I decided to keep it in our DB as a nullable integer.
  • I decided to keep it as null, not set a default value to 0, because I don't see any reason why someone wouldn't like to set 0 in that metadata.
• Added a string for formatting disc number with track number, because I assume that in RTL languages the order may be reversed.
• Also, added zero-padding to track number in case there's a disc number, to make it look prettier.
• To properly handle cases where there are no disc number, I've decided to:
  • While sorting, set Int.MAX_VALUE instead of null to make sorting consistent with default Kotlin behavior (null as the last value when sorting ascending).
  • While sending track number through bundle, to not lose information about null (it defaults int to 0), I also use a constant value to later replace it with null.

Before/After Screenshots/Screen Record

• Before:

• After:

Fixes the following issue(s)

• Fixes Support album containing several disks #18

Acknowledgement

• I read the contribution guidelines.