Give real instead of page-aligned buffer size to Gstreamer

Give real instead of page-aligned buffer size to Gstreamer

We handle all our buffers as page-aligned mmap'ed chunks, and so
their size is often larger than their contents. This breaks plugins
that insist on computing the buffer size from the caps we report.

Fix this by explicitly setting GST_BUFFER_SIZE for the final buffer
as the content size. This will break if we ever want to use this
camera source with devices that can do encoding on the v4l layer
or below.

Change-Id: I1b6d848162bfa9e06e729ebb7b6d986ba5a056ce