Destroy HidServiceLinux::FileThreadHelper before DeviceManagerLinux.
Destroy HidServiceLinux::FileThreadHelper before DeviceManagerLinux.

This change makes sure that an instance of HidServiceLinux's internal
FileThreadHelper class is destroyed before the DeviceManagerLinux it
depends on by registering it first as a MessageLoop destruction
observer.

This fixes a DCHECK introduced by my previous commit 87ac8da7e4f11e52a9.

BUG=None

Review URL: https://codereview.chromium.org/1315233002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345931}

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