Touch event latency metrics

Add a basic metric about touch event latency.

Metric design doc:
https://docs.google.com/document/d/1funj0dM9oZMiivhz43U5WM2wA2zWiPWsOxO-Jl5PmbY/edit#

Bug: 111431676
Bug: 122748138
Test: manual test following procedures in the doc
Change-Id: Ifc2643b53c2211f67355b27c2ab4e706c00eae97