Defer sending display events to cached apps

When the target app is in cached mode, the display events won't be sent
out until the app becomes non-cached later. Those redundant display
events can be collapsed (e.g. repeated display/brightness change) to
improve the efficiency.

Events belonging to different displays won't impact each other. They
will still reach the target apps in the original order.

Bug: 217609394
Test: atest DisplayEventTest
Test: logcat to confirm the interleaved display events are correctly
sent to the cached applications after they become non-cached

Change-Id: Icf25b87f2bf0b14b3f5a3f6df8fb42e1b599bba7