HWC2: Fix virtual displays

Close the output buffer release fence per the contract in
hwcomposer2.h.

Also changes the HWC2 device to hold a weak pointer to the display,
which allows it to properly remove it when it gets torn down.

Bug: 29945684
Change-Id: Ib1b5d81e42a63d0e65b189f7c6c62e661e0ab6ae
diff --git a/services/surfaceflinger/DisplayHardware/HWC2.h b/services/surfaceflinger/DisplayHardware/HWC2.h
index 8ab61e9..68781da 100644
--- a/services/surfaceflinger/DisplayHardware/HWC2.h
+++ b/services/surfaceflinger/DisplayHardware/HWC2.h
@@ -88,7 +88,8 @@
     // Other Device methods
 
     // This will create a Display if one is not found, but it will not be marked
-    // as connected
+    // as connected. This Display may be null if the display has been torn down
+    // but has not been removed from the map yet.
     std::shared_ptr<Display> getDisplayById(hwc2_display_t id);
 
     bool hasCapability(HWC2::Capability capability) const;
@@ -181,7 +182,7 @@
     HWC2_PFN_SET_LAYER_Z_ORDER mSetLayerZOrder;
 
     std::vector<Capability> mCapabilities;
-    std::unordered_map<hwc2_display_t, std::shared_ptr<Display>> mDisplays;
+    std::unordered_map<hwc2_display_t, std::weak_ptr<Display>> mDisplays;
 
     HotplugCallback mHotplug;
     std::vector<std::pair<std::shared_ptr<Display>, Connection>>