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
diff --git a/tests/tests/display/src/android/display/cts/BrightnessTest.java b/tests/tests/display/src/android/display/cts/BrightnessTest.java
index 6b01df2..f057b73 100644
--- a/tests/tests/display/src/android/display/cts/BrightnessTest.java
+++ b/tests/tests/display/src/android/display/cts/BrightnessTest.java
@@ -101,6 +101,10 @@
 
             grantPermission(Manifest.permission.BRIGHTNESS_SLIDER_USAGE);
 
+            // Sleep to work around a issue with events being recorded incorrectly
+            // on devices with sensors that require a warmup time.
+            Thread.sleep(2000);
+
             // Setup and remember some initial state.
             recordSliderEvents();
             setSystemSetting(Settings.System.SCREEN_BRIGHTNESS, 20);