Ensure response_sender always runs for Smart Dim D-Bus

Previously, the D-Bus response_sender was not run for calls from powerd
to Ash's Smart Dim prediction service, under certain conditions
* Smart Dim disabled by policy
* Smart Dim request canceled (typically due to starting another one
  while the first was pending, which is expected to happen sometimes)

Sending a response_sender is now enforced by runtime CHECK().

This CL ensures response_sender is always invoked as follows, using a
default response of false (corresponding to "allow screen dim to
proceed"):

* For Smart Dim disabled by policy, a trivial short-circuit is used to
  return false.

* For canceled requests, the interface of SmartDimMlAgent is updated so
  that it passes an optional<ModelPrediction> to the client instead of a
  ModelPrediction. When an empty optional<> is returned thus, the client
  now invokes the callback to powerd with a default value of false. When
  a non-empty optional<> is provided, the pre-existing logic updating
  Smart Dim's internal state, logging, etc takes place as before.

Bug: chromium:406621242
Change-Id: I595868fbc83458876490ec546653e2302d1b44c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6433508
Reviewed-by: Xinglong (Alan) Luan <alanlxl@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@google.com>
Cr-Commit-Position: refs/heads/main@{#1443084}