[PCCT] Hide spinner early when it can

Dynamically-resizing-window (a.k.a. window-above-navbar) version does
not have the UI flickering at the end of dragging action, so there is
no need to keep the spinner till the end. This CL hides it at earlier
time i.e. 500ms after showing it. It is long enough to hide the glitch
at the beginning that briefly exposes the host app screen.

Bug: 1334768
Change-Id: Ib974fa5f2587aba0bee28d995e06dac1101edde6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3843040
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1037997}