buffet: libchromeos: Move ExportedObjectManager to libchromeos We'd like to reuse this code in peerd. BUG=chromium:398626 TEST=buffet/libchromeos unittests Change-Id: I37be87dacbd5507d0f405b832cb45f65b4d70203 Reviewed-on: https://chromium-review.googlesource.com/211962 Reviewed-by: Christopher Wiley <wiley@chromium.org> Commit-Queue: Christopher Wiley <wiley@chromium.org> Tested-by: Christopher Wiley <wiley@chromium.org>