Move _select encoding and decoding to threads.

[noamraph]

This includes 3 separate changes:

1. In FlutterContactsPlugin.kt, encode the _select results in the same thread in which fetching is done, instead of on the main thread.
2. In flutter_contacts.dart, decode on a separate dart isolate.
3. In example/main.dart, start fetching the contacts after the user presses the button.

There is still non-responsiveness. I think it's shorter.

The change in FlutterContactsPlugin.kt is reasonable - it just follows the implementation of MethodCallHandler, which is pretty small.

The change in flutter_contacts.dart is pretty ugly. I'm really not sure it's worth it.

The change in example/main.dart was useful because it allowed me to start profiling and then press the button.

I send this mostly as reference. If I were the maintainer, I would see how much the change in FlutterContactsPlugin.kt shortens the non-responsive time, and decide if it's worth the (not too big) added complexity. I would not merge the change in flutter_contacts.dart.

I hope this helps,
Noam

[aytunch]

Hi, I know this PR is old, but it could help the Android performance a lot for devices which have many contacts.
Will this be worked on any time soon? Thanks.