Make chromeos/{dbus/}constants components

The constants subdirectories were inadvertently defined as source sets,
even though they are included by mulitiple libraries. This causes
ODR violations in asan component builds.

Sinces these are just constants it should not affect any runtime
behavior.

This CL also adds a BUILD.gn file to each subdir and cleans up a few
unnecessary dependencies.

For mechanical BUILD.gn changes:
[email protected]

Bug: 933063
Change-Id: I05195779ddd606c4ad33ae7c1c713044a8cb089e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1495862
Reviewed-by: Dan Erat <[email protected]>
Reviewed-by: Pavol Marko <[email protected]>
Commit-Queue: Steven Bennetts <[email protected]>
Auto-Submit: Steven Bennetts <[email protected]>
Cr-Commit-Position: refs/heads/master@{#637924}
32 files changed