Put drag-corner-to-resize-pip behind flag.

Instead of using the resource config, put drag-corner-to-resize-pip
behind the enable_desktop_windowing_pip flag.

Bug: 375054477
Test: Check that drag-corner-to-resize PiP is not enabled unless the
flag is enabled.
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I63a9f6a5793cf0c1823a2f02bdc0cde2408107da