Make sysprop for PiP2 flag persist

We are using a system property to easily
toggle the PiP2 flag during development.

However, the Core now caches the value
of the sysprop which means we have to do
adb reboot for flag change to take effect.
Hence the sysprop needs to be persistent.

Bug: 311462191
Test: adb shell setprop persist.wm_shell.pip2 1; adb reboot
Change-Id: Ia5ac78c1bdb41eaf28d7aaf727d17f3ff4225828