Mark DisplayManagerServiceTest#testDisplayedContentSampling as FlakyTest

Bug: 172555744
Test: None
Change-Id: I62dbf367d09c8f71d22d53fb0eeeda3681f03564

services/tests/servicestests/src/com/android/server/display/DisplayManagerServiceTest.java

@@ -460,6 +460,7 @@ public class DisplayManagerServiceTest {
      * Tests that collection of display color sampling results are sensible.
      */
     @Test
+    @FlakyTest(bugId = 172555744)
     public void testDisplayedContentSampling() {
         DisplayManagerService displayManager =
                 new DisplayManagerService(mContext, mShortMockedInjector);