HWComposer: Unset mPrimaryHwcDisplayId after disconnectDisplay

We should unset mPrimaryHwcDisplayId when disconnectDisplay()
is called for the primary display. This can happen during
display change and it will cause isHeadless() to be false.
This way SurfaceFlinger will call getPrimaryDisplayId()
which will crash.

Test: HWComposerTest
Test: libsurfaceflinger_unittest
Bug: 195900759
Change-Id: I3c00e3b864fef6f1b088ffae0539c1ad16b7492b