Switch to 3 framebuffer surface buffers

Switches from 2 to 3 buffers for SurfaceFlinger's FramebufferSurface.
This avoids an issue exposed by the HWC2 path where, since we now defer
buffer release, the GL driver ends up hanging on dequeueBuffer, which
is called in response to a glViewport command.

Bug: 62783122
Test: android.server.cts.ActivityManagerDisplayLockedKeyguardTests#
          testVirtualDisplayHidesContentWhenLocked

Change-Id: I6f68823c652fcfb12b16154653154ffe54457209
diff --git a/BoardConfig.mk b/BoardConfig.mk
index b25a996..2c78166 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -41,7 +41,7 @@
 TARGET_RELEASETOOLS_EXTENSIONS := device/google/dragon
 
 USE_OPENGL_RENDERER := true
-NUM_FRAMEBUFFER_SURFACE_BUFFERS := 2
+NUM_FRAMEBUFFER_SURFACE_BUFFERS := 3
 BOARD_DISABLE_TRIPLE_BUFFERED_DISPLAY_SURFACES := true
 BOARD_USES_DRM_HWCOMPOSER := true
 BOARD_DRM_HWCOMPOSER_BUFFER_IMPORTER := nvidia-gralloc