Implement SkiaGLRenderEngine::cleanFramebufferCache

On hotplug SF calls cleanFramebufferCache in order to
clean the existing handles to framebuffers. This CL
implements the function for SkiaGLRenderEngine and
fixes a bug where after each hotplug we end up with
+3 framebuffers.

Fixes: 182578852
Test: manually flash build, hotplug and call
      adb shell dumpsys SurfaceFlinger | grep Framebuffer
      to verify that only 3 framebuffers are present

Change-Id: I2a0d1ab6005e3c7a994f8c8c0eeec79afc96d109