[layertracegenerator] avoid allocating unnecessary buffers

namespace android::surfaceflinger {

class FakeExternalTexture : public renderengine::ExternalTexture {

    const sp<GraphicBuffer> mEmptyBuffer =

            sp<GraphicBuffer>::make(1u, 1u, PIXEL_FORMAT_RGBA_8888,

                                    GRALLOC_USAGE_SW_WRITE_OFTEN | GRALLOC_USAGE_SW_READ_OFTEN);

    const sp<GraphicBuffer> mEmptyBuffer = nullptr;

    uint32_t mWidth;

    uint32_t mHeight;

    uint64_t mId;