Implementing detector of view flashes

I.e. when a view appears or disappears for a short time.

Moving some common parts of AlphaJumpDetector and FlashDetector to their parent class, AnomalyDetector, and moving AnomalyDetector to a separate file.

Also tweaking the code a bit.

Flag: N/A
Test: presubmit, local runs
Bug: 286251603
Change-Id: I022e68eb90147abd3ed4ee3b285d672bb19c997d