Enable IME test: "show and hide IME layer"

This test was previously disabled because it was flaky. This CL re-enables the test.

 To check if the IME works correctly the flicker tests check:
 (1) IME layer is invisible in the beginning
 (2) IME layer is displayed
 (3) once shown, IME layer is visible in all following entries

Test: atest FlickerTests:OpenImeWindowTest
Change-Id: I89abb369904fc39733f4aef8038e2ad1be2607d9

tests/FlickerTests/src/com/android/server/wm/flicker/OpenImeWindowTest.java

@@ -24,7 +24,6 @@ import androidx.test.filters.LargeTest;
 import com.android.server.wm.flicker.helpers.ImeAppHelper;
 
 import org.junit.FixMethodOrder;
-import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.MethodSorters;
@@ -62,7 +61,6 @@ public class OpenImeWindowTest extends NonRotationTestBase {
                 .forAllEntries());
     }
 
-    @Ignore("Flaky. Pending debug")
     @Test
     public void checkVisibility_imeLayerBecomesVisible() {
         checkResults(result -> LayersTraceSubject.assertThat(result)