Reduce number of copies into unmapExternalTextureBuffer

Make unmapExternalTextureBuffer take in an rvalue of sp<GraphicBuffer>,
which reduces the number sp copies.

Bug: 215661436
Test: builds
Change-Id: I4f9b6e3d6863ce6815810fdda038dc9e730f0b44
diff --git a/libs/renderengine/ExternalTexture.cpp b/libs/renderengine/ExternalTexture.cpp
index 84771c0..210dca5 100644
--- a/libs/renderengine/ExternalTexture.cpp
+++ b/libs/renderengine/ExternalTexture.cpp
@@ -39,7 +39,7 @@
 }
 
 ExternalTexture::~ExternalTexture() {
-    mRenderEngine.unmapExternalTextureBuffer(mBuffer);
+    mRenderEngine.unmapExternalTextureBuffer(std::move(mBuffer));
 }
 
 } // namespace android::renderengine::impl