Use shade dialog context to open channel editor dialog

This is needed as the shade is now using a TYPE_NOTIFICATION_SHADE window context that doesn't support opening dialogs.

Bug: 362719719
Bug: 383294128
Bug: 397974982
Test: ChannelEditorDialogControllerTest
Flag: com.android.systemui.shade_window_goes_around
Change-Id: I7bc07b3d3c83c5670f01ebcdf7d0bab25fe2236c