Revert "Close FileChannels in UpdateHandler#copyFile."

This reverts commit 3e267026.

It turns out that the behavior change in libcore was unintentional, and
it was already fixed [1].  Let's revert our workaround back to see if
the existing code is compatible with N.

  [1]: Ibacb192abc37870c74a2500d65b94d68f9c2318e
       5e7b572c2b494ab86ddd2baca3883a40a6064c1e

Bug: 26239281
Change-Id: I6cd2340492d93251231e7ee37c3d4f82c1721293