Avoid a potential race condition on mDisplays

The race could occur when transitioning in/out of VR flinger mode.
It is now avoided by ensuring that the primary |DisplayDevice| is always
created once |mStateLock| is released, and ensuring that all accesses
to the primary |DisplayDevice| are guarded by |mStateLock|.

Bug: 36194616
Test: Normal behavior is unchanged. Explicitly testing the race
condition required instrumenting code with sleep statements.

Change-Id: I1f8fcf23982c311d47267beca8127b6c29562ce5
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index 09434f6..32739b6 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -206,7 +206,7 @@
         }
         disp = DisplayDevice::DISPLAY_EXTERNAL;
     }
-    mEventHandler->onHotplugReceived(disp,
+    mEventHandler->onHotplugReceived(this, disp,
             connected == HWC2::Connection::Connected);
 }