Keep ParcelFileDescriptor alive and call Close() when done

When native code writes to a Content-URI file opened by Java code,
it must keep the ParcelFileDescriptor alive and call Close() when
done in order to correctly communicate with any ContentProvider
which has a close listener on the PFD.

Previously, when calling ParcelFileDescriptor.detachFd(), any
listening provider would be notified that the file has closed.
In the case of the DownloadProvider, it considers that the file
has finished writing at that point, and it will stat() the file
and update its database with the current file size.  This creates a
race condition where writes by the native code will update the
file on disk, but the provider and any apps such as DocsUI may show
the file empty or smaller than it really is.

Bug: 413207407
Change-Id: Ie29a002c2669ccdcb0b082845855330ad6455ddd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6508685
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1456759}