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