size IMemoryHeap properly for screenshots

since we're using glReadPixels(), we only need to use
the width (as opposed to the stride) of the source
screenshot.

Bug: 8374664
Change-Id: I145c80f4fff5444df7c77c4f52e70a7203caddbd

services/surfaceflinger/SurfaceFlinger.cpp

@@ -2787,7 +2787,7 @@ status_t SurfaceFlinger::captureScreenImplLocked(
             sp<GraphicBuffer> buf(consumer->getCurrentBuffer());
             sw = buf->getWidth();
             sh = buf->getHeight();
-            size_t size = buf->getStride() * sh * 4;
+            size_t size = sw * sh * 4;
 
             // allocate shared memory large enough to hold the
             // screen capture