Use sysproperties to check PiP2 flag for dev

Use system properties to check for the "PiP2 flag"
for easier and quicker testing and development.

This is meant to be reverted once before we move on to
teamfooding stages as a part of trunk-stable.

Bug: 311462191
Test: adb shell setprop wm_shell.pip2 1;
      adb shell am crash com.android.systemui
Change-Id: I11630369715fd02be50d9cc9aa1f15c47e44815a