Background mode did not work

[duyhungtnn]

Problem

• Our SDK has a background task feature to load the evaluation in the background mode.
• But our example code & document didn't mention how to configure the background task in a correct way
• The background task handler did not register anywhere in the code.
• From iOS 14, we need to register all background task handlers before app finishes launching

Expectation

• It is necessary to update the example App to showcase the utilization of the background task.
• We require a section on the document that discusses background tasks.
• It will help other people who want to use it.
• The background mode should work after the developer enable it

[duyhungtnn]

closed by #28