Fix Android IO and Worker threads priority issue

[rogeryi]

Manually set IO and Worker threads priority to THREAD_PRIORITY_LESS_FAVORABLE(1).

1. From Android 8+, the priority of main thread (aka Flutter's platform thread) has been raised from THREAD_PRIORITY_DEFAULT(0) to THREAD_PRIORITY_VIDEO(-10) by system;
2. The UI/Raster/IO and Worker threads are created by platform thread and inherit its priority;
3. Flutter raised priority of UI and Raster threads to -1 and -5, but leave IO and Worker threads untouched;
4. The current result is IO and Worker threads have extreme high priority (-10), much higher than UI and Raster thread;
5. The image decoding and mipmap generation on IO and Worker threads are very time consuming and usually block the UI and Raster threads when App loading bunch of images, especially on low end devices;

So manually set IO and Worker threads to an appropriate background priority such as THREAD_PRIORITY_LESS_FAVORABLE(1) will be a good move, it will work nicely on both Android 8+ and lower version.

List which issues are fixed by this PR. You must list at least one issue.

The background threads - IO and Worker threads, have much higher priority than UI and Raster thread, and easy to make them blocked and cause jank.

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[rogeryi]

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[chinmaygarde]

cc @dnfield. This seem like a sound patch. Though, we should followup on an iOS implementation as well.

[dnfield]

This does not impact the Dart worker threads, which should likely get similar prioritization.

[dnfield]

(not that it should block this patch)

[rogeryi]

This does not impact the Dart worker threads, which should likely get similar prioritization.

Dart worker inherit Flutter UI thread's -1 priority, lower it to 1 maybe better, although not as necessary as this

[chinmaygarde]

We should figure out a way to generalize this to all platforms. In the meantime, I think this is fine for Android.