Add delay of warmup time to test brightness slider tracking

On our devices, Brightness slider cts is failed frequently.
The failure is caused by warmup time of light sensor.
Some models have long warmup time for lighr sensor and should be set 100~400ms to config.
A delayed light sensor event intercept the value to be asserted.

BrightnessTest#testBrightnessSliderTracking

In order to make them get passed clearly, Add enough delay for warm-up time..
Test: cts-tradefed run cts -o -m CtsDisplayTestCases -t android.display.cts.BrightnessTest#testBrightnessSliderTracking
Signed-off-by: Taehwan Kim <taehwan.kim@lge.com>

Change-Id: I1e4c2e531777017ad5acf0937d610710ce73fd89