Do not flush when sending view tree appearing event.

This can mitigate the IPC spam by half, since the view tree appearing
event always comes with a view tree appeared event, and we just need to
flush at the view tree appeared time.

BYPASS_INCLUSIVE_LANGUAGE_REASON=existing API

Bug: 269435271
Change-Id: I0ba3f7df7d6b511d155ac9509826d39633b668f6
Test: some unit tests & I've manually tested it locally.