2/ Initial AppPairs flicker test case

1. Send adb command to pair test apps
2. Assert primary & secondary apps visibility
3. Assert Region of primary & secondary apps in portrait

Bug: 172776659
Test: atest WMShellFlickerTests:AppPairsTest
Test: set TEST_REPETITIONS = 10, tests no flaky
Change-Id: If41340c82782da01909d84b9ad101556e3234aa1
5 files changed