[Files app] Fix MyFiles to auto expand only once

Fix MyFiles to only auto-expand once (in its constructor), this fixes
issue where MyFiles will expand when other roots were selected.

MyFiles to have children before collapsing.

Deflake: Refactor the expansion to be only at the constructor and wait
Change-Id: Id90d4ffa5537f21693818938337a920cd6acaaf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1510252
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638873}
4 files changed