[PiP on CD] Use PipFlags.isPip2ExperimentEnabled in PipDesktopState

instead of checking the PiP flag value directly. This ensures that
PipDesktopState#isConnectedDisplaysPipEnabled returns true if Pip2 flag
value is false, but PiP on DW flag is true.

Bug: 416341513
Test: Manual - verify that we can enter PiP on CD
Test: atest PipDesktopStateTest
Flag: com.android.window.flags.enable_connected_displays_pip
Change-Id: Ia5534f8d61ee18a2148f0ba08adeb15467511033