SF: Clean framebuffer cache on hotplug

This CL cleans the framebuffer cache when a hotplug event
for already connected display is processed. This way when we
destruct the DisplayDevice the framebuffers will be completely
deallocated, so they can later be reallocated.

Test: 1. unplug HDMI
      2. adb shell dumpsys SurfaceFlinger | grep FramebufferSurface
         verify only 3 framebuffers are allocated
      3. plug HDMI
      5. adb shell dumpsys SurfaceFlinger | grep FramebufferSurface
         verify only 3 framebuffers are allocated
Test: Using dev SurfaceFlinger backdoor (1036) which injects hotplug:
      adb shell service call SurfaceFlinger 1036 && \
        adb shell dumpsys SurfaceFlinger | grep FramebufferSurface
Bug: 160112047
Bug: 167670568
Change-Id: I96282bec439ebf413dcd5d57f6b884eccef19c5f
diff --git a/libs/renderengine/gl/GLESRenderEngine.h b/libs/renderengine/gl/GLESRenderEngine.h
index 2a98bd8..8f0df2c 100644
--- a/libs/renderengine/gl/GLESRenderEngine.h
+++ b/libs/renderengine/gl/GLESRenderEngine.h
@@ -135,6 +135,7 @@
     status_t bindFrameBuffer(Framebuffer* framebuffer);
     void unbindFrameBuffer(Framebuffer* framebuffer);
     void bindExternalTextureImage(uint32_t texName, const Image& image);
+    void cleanFramebufferCache() EXCLUDES(mFramebufferImageCacheMutex) override;
 
     // A data space is considered HDR data space if it has BT2020 color space
     // with PQ or HLG transfer function.