Comparable dbus_utils::Variant

This change is required for [1].
- Add Variant::operator==. Required for detecting when DBus
  properties have changed. Also useful for comparison in tests.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/7006695

Change-Id: I87d75c2741fb7ebaa6da41e333277b09dbbd95f4
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7007276
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526718}