Turn on PiP2 if ENABLE_DESKTOP_WINDOWING_PIP is ON

If ENABLE_DESKTOP_WINDOWING_PIP is enabled via Developer options or
etc., override the PiP gantry flag to be ON. This assumes the
ENABLE_DESKTOP_WINDOWING_PIP flag never advances ahead of PiP2.

Flag: com.android.wm.shell.enable_pip2
Bug: 409622499
Test: adb shell dumpsys activity service SystemUIService WMShell
Change-Id: Iac6bd237829b1b36ae27feed6a49042d1c96f2db