Modified FlautoRecorderEngine.java to re-use buffers and avoid GC churn

[781flyingdutchman]

Hi, really appreciate your flutter_sound package, but per this issue when using the toStream option it generates massive Garbage Collection churn on Android, see this log:

2026-01-25 01:19:47.746 27156-27156                 I  Waiting for a blocking GC Alloc
2026-01-25 01:19:47.762 27156-27161                 I  Background young concurrent mark compact GC freed 104MB AllocSpace bytes, 0(0B) LOS objects, 0% free, 255MB/255MB, paused 413us,1.215ms total 302.740ms
2026-01-25 01:19:47.762 27156-27185                 I  WaitForGcToComplete blocked ProfileSaver on Background for 236.711ms
2026-01-25 01:19:47.762 27156-27156                 I  WaitForGcToComplete blocked Alloc on Background for 16.343ms
2026-01-25 01:19:47.762 27156-27156                 I  Starting a blocking GC Alloc
2026-01-25 01:19:47.815 27156-27156                 I  Alloc concurrent mark compact GC freed 250MB AllocSpace bytes, 0(0B) LOS objects, 81% free, 5448KB/29MB, paused 132us,1.471ms total 52.776ms
2026-01-25 01:19:47.815 27156-27161                 I  WaitForGcToComplete blocked Background on Alloc for 49.116ms
2026-01-25 01:19:48.455 27156-27156                 I  Waiting for a blocking GC Alloc
2026-01-25 01:19:48.498 27156-27161                 I  Background concurrent mark compact GC freed 250MB AllocSpace bytes, 0(0B) LOS objects, 81% free, 5460KB/29MB, paused 153us,1.360ms total 199.971ms
2026-01-25 01:19:48.498 27156-27156                 I  WaitForGcToComplete blocked Alloc on Background for 42.261ms

That is 604MB collected, causing 107ms of blocked Alloc in less than 1 second (and it goes on forever).

The reason is that in FlautoRecorderEngine.java you allocate buffers within a tight loop. I did not have time to look into this in detail, but this PR modifies FlautoRecorderEngine.java to use a re-usable buffer that does not have to be allocated frequently, and as a result the blocking GC Alloc calls are gone completely.

Because flutter_sound depends on this repo it is hard to make the change there, but perhaps you can adopt the changes in this PR (or something similar) so that the flutter_sound package becomes much more performant on Android.

Thanks!

[Haotian9850]

+1 to this issue. Frequent GC causes app jitter and (now that flutter has merged the platform and UI threads) it's even more of a performance issue

[781flyingdutchman]

Following the initial commit to this PR I have made further changes to address runtime errors in both Android and iOS code bases, that caused app crashes for various reasons. Still not 100% convinced all nullpointer issues in iOS are fixed, but I do think it is getting better