Add display related tests in AccessibilityManagerServiceTest

With this change, the test starts to use a real instance of
AccessibilityDisplayListener. Because DisplayManager class is a final
class and is hard to mock in service tests, this introduces a wrapper
for it.

Bug: 400872190
Test: AccessibilityManagerServiceTest
Flag: TEST_ONLY
Change-Id: I7b425727110ede3ca0601b6dc6c7acd85bb91a75