[fake-display] Remove FakeDisplayStack from unit tests.

FakeDisplayStack attaches a Display Coordinator to the FakeDisplay
driver implementation. This introduces the following problems.

1. Some tests replace the Coordinator's event listener with their own,
   in order to catch OnCaptureComplete events.  This makes the tests
   difficult to reason about, because the Coordinator does not support
   having its event listener removed, and the driver does not support
   having its event listener replaced.

2. This is an unnecessary increase in the test surface area, because
   FakeDisplay only depends on Sysmem and on the Driver Runtime.

This CL replaces the FakeDisplayStack with the parts that are used in
the unit test.

Bug: 388885807
Test: fx test //src/graphics/display/drivers/fake
Change-Id: I1782768424b83cb691b056ea6662609352c563dd
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1233747
Commit-Queue: Victor Costan <[email protected]>
Fuchsia-Auto-Submit: Victor Costan <[email protected]>
Reviewed-by: Yilong Li <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

Author: pwnall