Allow a primary display disconnect

This patch forwards the primary display disconnect event to the
Framework, and otherwise ensures that SurfaceFlinger does not crash
while there is no primary display.

Note that the Framework does not yet accept this change. In particular
the ActivityManager ActivityStackSupervisor code promptly asserts that
one cannot remove the primary display. With this assertion disabled, the
framework does not crash (surprisingly). And if the Framework
subsequently receives a primary display connect event, it does not seem
to do anything useful -- the display remains in a default off state, and
no layer stack/viewport/etc is set on it.

Bug: 38464421
Test: Works (with workarounds as noted) on a Chromebook
Test: Added Unit test passes on Pixel 1 XL

Change-Id: Ia11439030efdc53bc17474b71a0ffb3d3085bb49