Load archives in background.

ArchiveProvider's queryChildDocuments used to be synchronous, which
would for large archives may take seconds and timeout.

This CL moves loading to another thread and utilizes EXTRA_LOADING so
DocumentsUI has a chance to show a spinner when loading large archives.

Bug: 31783726
Test: Open a ZIP file containing Linux kernel.

Change-Id: I91a97a12e22db71db77d5d06d66d108a03622c3f