Adding two UiEvent IDs to log when slider tracks and stops tracking

touch.

The events are part of a new enumerable class BrightnessSliderEvent.
They are: SLIDER_STARTED_TRACKING_TOUCH and SLIDER_STOPPED_TRACKING_TOUCH

Test: atest BrightnessSliderControllerTest
Test: manual, make statsd_testdrive && statsd_testdrive 90
Bug: 295932558

Change-Id: I8062a3950b67d218aba224b08cf12ced864db33e