Remove unused include in device_management_service.h

Should save ~164 MiB of compiler input size.

Adds missing includes and forward declarations to fix the build.

Also moves the implementation of
`DeviceManagementService::GetTaskRunnerForTesting` to the .cc file so
that the existing forward declaration of `SequencedTaskRunner` in the
header is effective.

Bug: 40318405
Change-Id: Idbc1c857df93abf3cf60d6d138981dbd7d9aaf23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6781637
Commit-Queue: David Sanders <dsanders11@ucsbalum.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491689}