Move frameworks users over to libcore hex encoding API.

Removes dependency on apache-http.

bug: 18027885
Change-Id: I508db016747985ecdd68e02d5d4efcd0ddd694df
Throw IllegalArgumentException on invalid hex-strings.

Beats returning null since there's let's chance it will be interpreted
as the lack of a password.

Change-Id: I4986a8e806d9066129f696ab9f2e80655424e723