[code health] Removing "dangling" label from `exported_object_` field.

This CL removes the `AcrossTasksDanglingUntriaged` annotation from the
`DbusProperties::exported_object_` field.  Reordering of destruction of
fields of `SystemMediaControlsLinuxTest` helps to prevent that pointer
from dangling.

Fixed: 41485221
Change-Id: Ib9bc7475762d5d9269056844a8465015776f59f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6481222
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1450270}