Deflaky BackGestureInvokedTest#testImeDismissed test

This test is verify the IME window should be dismissed when receive
back gesture, so there should wait for IME window show before trigger
back gesture.

Bug: 293699647
Test: atest BackGestureInvokedTest#testImeDismissed --iterations 50
Change-Id: Ifdfe519b47526393697b530b78e4e9b1a651bab4
diff --git a/tests/framework/base/windowmanager/src/android/server/wm/backnavigation/BackGestureInvokedTest.java b/tests/framework/base/windowmanager/src/android/server/wm/backnavigation/BackGestureInvokedTest.java
index 6f98f2e..440f782 100644
--- a/tests/framework/base/windowmanager/src/android/server/wm/backnavigation/BackGestureInvokedTest.java
+++ b/tests/framework/base/windowmanager/src/android/server/wm/backnavigation/BackGestureInvokedTest.java
@@ -154,6 +154,7 @@
         launchActivity(componentName);
         mWmState.waitForActivityState(componentName, STATE_RESUMED);
         mWmState.assertVisibility(componentName, true);
+        mWmState.waitAndAssertImeWindowShownOnDisplay(DEFAULT_DISPLAY);
 
         triggerBackEventByGesture(DEFAULT_DISPLAY);