Populates the notificationCount field in DisplaySwitchLatencyEvent.

The change injects ActiveNotificationsInteractor into DisplaySwitchLatencyTracker and uses its allNotificationsCountValue to capture the number of notifications when the switch event begins.

Added a unit test to DisplaySwitchLatencyTrackTest that mocks ActiveNotificationsInteractor. The test verifies that the notification count provided by the mock is correctly recorded in the final logged event.

Test: atest DisplaySwitchLatencyTrackerTest
Bug: 304491690
Flag: EXEMPT minimal changes
Change-Id: I9288900d12c90804d933ca401de38915e46b8fb4