Use shared session bus for MPRIS

This removes the restriction of GetSharedSessionBus() that clients
requiring named sessions should use their own bus. I originally made the
comment in dbus_thread_linux.h because I thought a connection could only
have one unique name, however connections may have multiple names. The
only client (MPRIS) using dbus_thread_linux::GetTaskRunner() is changed
to use GetSharedSessionBus(). The last caller of GetTaskRunner() is
removed, so the function is removed from the public API.

Change-Id: I70cdcf0c34a570e250424be9c556919dc00673ec
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6776165
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1489941}