Fix empty view at trace load time

The issue was caused by the TimelineCoordinator not notifying
the timestamp observers when the current active trace types are set.

Added unit test for TimelineCoordinator.

Fix: b/258212101
Test: npm run build:unit && npm run test:unit
Change-Id: I32323f3a82da12f5956e11f84c774c6b6142bc5f
4 files changed