Wait "launch_second_activity" button to be gone

When running on slow devices (e.g. cuttlefish), the animation may take some time to start. The previous checks would pass if the animation didn't start and this causes an exception on the flicker tests

Test: atest FlickerTests
Change-Id: I8de4410e1c169d90d69d9a440daa82ec005fe222

tests/FlickerTests/src/com/android/server/wm/flicker/helpers/TwoActivitiesAppHelper.kt

@@ -37,16 +37,21 @@ class TwoActivitiesAppHelper @JvmOverloads constructor(
         .launcherStrategy
 ) : StandardAppHelper(instr, launcherName, component, launcherStrategy) {
     fun openSecondActivity(device: UiDevice, wmHelper: WindowManagerStateHelper) {
-        val button = device.wait(
-                Until.findObject(By.res(getPackage(), "launch_second_activity")),
-                FIND_TIMEOUT)
+        val launchActivityButton = By.res(getPackage(), LAUNCH_SECOND_ACTIVITY)
+        val button = device.wait(Until.findObject(launchActivityButton), FIND_TIMEOUT)
 
         require(button != null) {
             "Button not found, this usually happens when the device " +
                     "was left in an unknown state (e.g. in split screen)"
         }
         button.click()
+
+        device.wait(Until.gone(launchActivityButton), FIND_TIMEOUT)
         wmHelper.waitForAppTransitionIdle()
         wmHelper.waitForFullScreenApp(component)
     }
+
+    companion object {
+        private const val LAUNCH_SECOND_ACTIVITY = "launch_second_activity"
+    }
 }