Prepare .wm.shell.flicker for test migration

Move common constants to a separate class.
Rename abstract FlickerAppHelper to BaseAppHelper.
Give BaseAppHelper a UiDevice (from androidx).
Make BaseAppHelper resposiblt for creating launcher.
Teach BaseAppHelper to launch app via intent and force-stop it.
Teach PipAppHelper to properly close Pip on TVs and work around
hasPipWindow() method not working on tv.

Bug: 171520419
Test: atest WMShellFlickerTests
Change-Id: Icc102a51351605b4adb27c3a1c2d73030eac33ff