Add audio playback on Linux

[matthiasn]

Resolves #1510 by replacing [just_audio](https://pub.dev/packages/just_audio) with media_kit since the former did not support Linux.

[codecov-commenter]

Codecov Report

Merging #1511 (951c953) into main (05b8088) will decrease coverage by 0.07%.
The diff coverage is 35.29%.

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@@            Coverage Diff             @@
##             main    #1511      +/-   ##
==========================================
- Coverage   70.90%   70.84%   -0.07%     
==========================================
  Files         256      256              
  Lines       10817    10811       -6     
==========================================
- Hits         7670     7659      -11     
- Misses       3147     3152       +5     

Impacted Files	Coverage Δ
lib/pages/settings/sync/sync_assistant_page.dart	1.09% <0.00%> (+0.01%)	⬆️
lib/sync/inbox/process_message.dart	50.00% <0.00%> (-3.66%)	⬇️
lib/widgets/sync/imap_config_status.dart	100.00% <ø> (ø)
lib/blocs/audio/player_cubit.dart	12.06% <12.50%> (-6.58%)	⬇️
lib/themes/theme.dart	70.27% <100.00%> (-0.79%)	⬇️
lib/widgets/sync/imap_config_form.dart	100.00% <100.00%> (ø)

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[alexmercerind]

media_kit bundles libmpv within media_kit_libs_macos_audio, by the way.
I'm not sure how if using external version will be a good idea.

---

Also... silly me. I forgot passing this argument in MediaKit.ensureInitialized.
What a goofy bug. I'm adding tests. It'll be addressed in next update.

[matthiasn]

Ah nice, that means I can remove that hack after updating the lib? Awesome, thanks for fixing it, felt like an odd hack having to do that to make the tests pass. Should have filed an issue right away :]

[alexmercerind]

We're reducing the minimum supported macOS version (if it matters) to improve backwards compatibility:
media-kit/libmpv-darwin-build#7

[alexmercerind]

Also, may I suggest --split-per-abi in Android build?

It can make huge difference. In current APK:

• There doesn't seem to be any compression in APK (compression ratio is 0).
• x86 native libraries are also bundled.
  • Flutter only supports arm64-v8a, armeabi-v7a & x86_64.
  • Dart can't compile to x86 in AOT (release) mode. So, there's no point of including those x86 libraries in release build.
  • media_kit & sqlite3 offer x86 binaries to support debug-mode for emulators running on AMD devices.

For each architecture, say arm64-v8a, media_kit adds about 5 MB to provide a really strong (in terms of codec support & performance).

Current 130 MB is undoubtedly very large. Out of which, most can be reduced with compression (possibly automatically done with --split-per-abi), 14 MB is redundant x86 binaries. As far as my approximation goes, --split-per-abi should give acceptable 18-25 MB APK for each architecture.

[matthiasn]

@alexmercerind thanks a lot, yeah I noticed that the APK had grown quite substantially recently after adding media_kit, from 83.8 MB last week, but I didn't have time to investigate yet. Will try out your suggestion now.