adb: fix directory creation logic.

Previously, for `adb pull $remote $local`, we would do the equivalent of
mkdir -p on `dirname $local`. This patch changes the behavior to only
creating directories that are being pulled, like scp.

Bug: http://b/27362811
Change-Id: I79f975ee9f2d9bc9e8be6a7c4f2de6d7ae2d2d23