adb: don't pull symlinks.

Pulling symlinks was broken for directories, and it doesn't seem like
there's a good way to make it not broken, given that the protocol doesn't
have readlink (and we don't want to create symlinks on Windows, anyway).
The behavior for files doesn't seem to be especially useful, either.

Bug: http://b/25601283
Change-Id: Ie1d27e93dd09cbc0c178623b390041d1cb11f726