Promote stable PIP app tests to presubmit

These flaky tests have been stable for the past week and can be promoted

Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip
Bug: 171049720
Bug: 167521849
Change-Id: Ibd78e9d9e53a5712ca781b265b1b65c830a74c08