Init displays to null layer stack

When a display is added, initialize it to use an empty layer stack, so
if it is somehow visible it will show black. It will be assigned the
real layer stack -- along with a projection and other properties -- by
window manager soon. Normally a display remains blanked until window
manager has decided what to show on it, but for HDMI connected at boot
that isn't currently the case.

Bug: 7258935
Change-Id: Ic9bb25f7a9b8d9d3772b097ab1d6fa03bc8780a1