SF: Fix for a leaked mock reported in processesVirtualDisplayAdded

I'm not sure why it only started being reported  recently, but removing
the injected virtual display from the SurfaceFlinger current/drawing
state fixes the leak.

Bug: None
Test: adb shell libsurfaceflinger_unittest
Change-Id: Ibaf1a8878e296707b9fbbd0a647908cc1205cdc2