stagefright: Fix SurfaceMediaSource getting handle from wrong position issue

In function passMetadataBuffer_l, the bufferHandle(ANativeWindowBuffer) is
saved to data (VideoNativeMetadata) but in function getMediaBufferHandle it
gets the bufferHandle from (MediaBuffer*)buffer->data() + 4, which is a wrong
position. To solve this problem, we should get handle from ANativeWindowBuffer,
not from buffer->data() + 4. (If get bufferHandle from buffer->data() + 4, the
function signalBufferReturned will print "returned buffer was not found in the
current list" error.

Test: Running wifi display, we can see the handle could be found in buffer list.

Change-Id: I71ecf9e2bca1db67d8d6e862ac16b07e939bf521
Signed-off-by: zhangbo_a <zhangbo_a@pinecone.net>
Signed-off-by: DennySPb <dennyspb@gmail.com>