Create a unit test for ModeChangeObserver class

This tests that the DisplayEventReceiver and
DisplayListener callbacks that ModeChangeObserver
overrides are being called correctly and are updating
the rejected modes vote without any race condition.

Bug: 401032713
Test: atest -c DisplayServiceTests:com.android.server.
display.mode.ModeChangeObserverTest
Flag: com.android.graphics.surfaceflinger.flags.display_config_error_hal

Change-Id: I33b98ce01d8a49d37f90e5fea021058b9d919850
Signed-off-by: Manasi Navare <navaremanasi@google.com>