[display] Propagate change state on display config update

This CL updates OnDidProcessDisplayChanges() to propagate a struct
of changes to the display configuration. This API is currently only
implemented by ChromeOS Ash.

There are several DisplayManager methods that modify the existing
display configuration and these methods can call each other in a
nested fashion. BeginEndNotifier exists to allow a single call to
OnWillProcessDisplayChanges() and OnDidProcessDisplayChanges() on
either side of an atomic change in display configuration state.

This CL introduces DisplayManager::PendingDisplayChanges which is
intended to track all display config changes that occur before
the last BeginEndNotifier falls out of scope. At this time
this is translated to a DisplayConfigurationChange struct more
appropriate for observers and propagated in the call to
OnDidProcessDisplayChanges().

This CL also adds a test to ensure what is propagated by
the DisplayManagerObserver is consistent with what is propagated by
the DisplayObserver.

Follow up CLs will also
  - update exo to leverage this as a means to consume an atomic
    update to the display configuration state.

  - consolidate the other existing mechanisms that communicate
    display configuration changes. For e.g. see
    DisplayManager::Delegate::Pre[Post]DisplayConfigurationChange()
    and downstream methods.

  - update clients that rely on other means to infer display
    configuration changes (using OnDisplayAdded[Removed]() for e.g.)

Bug: 1427019, 1408304, 1498683
Change-Id: I87bdf62b5fe90b7160c03e1f4597059288173959
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4973206
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1224481}