SF: Remove obsolete HWComposer members

Remove mExternalHwcDisplayId, an obsolete special case for legacy multi-
display mode. Assert primary display existence instead of propagating to
callers, which is closer to how headless mode would work, i.e. injecting
a placeholder primary display. Remove isVirtual checks now that they are
enforced at compile time. Prevent primary display disconnection for now.

Bug: 182939859
Test: libsurfaceflinger_unittest
Change-Id: I507db6a3ac0bda93005a86b38cca6ebbcd5c0155