Disable failing pip assertion

Test is very flaky in presubmit. Disable it until we can identify what is causing the failure. Currently sponge cannot fetch tests results and, thus, cannot be debugged.

Bug: 193436187

Test: atest WMShellFlickerTests
Change-Id: Ieb7a80d0675aefe1a80d10a0f8b512506d063bba

libs/WindowManager/Shell/tests/flicker/src/com/android/wm/shell/flicker/pip/EnterExitPipTest.kt

@@ -16,6 +16,7 @@
 
 package com.android.wm.shell.flicker.pip
 
+import android.platform.test.annotations.Postsubmit
 import android.platform.test.annotations.Presubmit
 import android.view.Surface
 import androidx.test.filters.RequiresDevice
@@ -66,7 +67,7 @@ class EnterExitPipTest(
         }
     }
 
-    @Presubmit
+    @Postsubmit
     @Test
     fun showBothAppWindowsThenHidePip() {
         testSpec.assertWm {