android: Fix importing pexpect on Mac

We should use the version of pexpect from pylib to make sure we can
import it an all platforms where it is available.

BUG=304337
R=torne@chromium.org

Review URL: https://codereview.chromium.org/26212003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227946 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227946