Reorganize PiP flicker code, move apps tests to new target

Moving the shared abstract classes to common subdirectory and moving
the 1P/3P apps related code to apps subdirectory.

Created a new build target for the apps tests so that they can be run
manually on demand separately, and also prevent running them with every
presubmit/postsubmit run.

Bug: 282758103
Test: atest WMShellFlickerTestsPip
Test: atest WMShellFlickerTestsPipApps
Change-Id: I431499a08ad4dbd15e2b83e71af643e411c706b9