Ensure apps not running on cold launch test

Currently the only test is not stopping existing apps and may be affected by previous runs on the same device.

Now, ensure all apps are closed when launching the test

Bug: 167521849
Test: atest FlickerTests:com.android.server.wm.flicker.launch
Change-Id: Ib29fb4835cd728e5855ae66452ff1ac9333499ed