Fix shade context used by VPN bar in QS to create a dialog

This is a temporary fix for the "VPN" dialog to make it use the shade dialog context instead of the plain QS one.

Note this issue is already fixed by the
com.android.window.flags.enable_window_context_override_type flag, but it will need some time to ramp up. This cl prevents crashes in the meantime.

Bug: 362719719
Bug: 407690449
Test: QSSecurityFooterTest
Flag: com.android.systemui.shade_window_goes_around
Flag: com.android.window.flags.enable_window_context_override_type
Change-Id: Ic02bed3672bdd2dbb227029f266033a601d013f5