Refactor DisplayManagerService

This is a do-no-harm refactor of DisplayManagerService in preparation
for a change that defers sending display events to frozen processes.

This change also ports the CTS DisplayEventTest to a similarly-named
unit test.

Flag: EXEMPT refactor
Test: atest
 * CtsDisplayTestCases
 * DisplayEventTest
Bug: 326315985
Change-Id: I60ac877235939342300a851465220027f07a04b9