Summary: Adds close button to the dialog box which appears while exporting a flipper trace. To implement this I had to first see what is the bottle neck of the export process. The major bottle neck turned out to be the serialization step. So to make the export process interruptible, I have put in a call `await idler.idle()` which resolves when the main thread is idle. I have also added the tests for the idler.
Reviewed By: passy
Differential Revision: D16183582
fbshipit-source-id: 4ec0c985e216fd9d41e91cdcd8b4cca66d2cb04d
7f2709e1a5