dkps: add Widevine keybox XML parser

Each DRM key issuing organization gives their DRM keys in different format. This
parser is able to parse the Widevine keybox XML file into python list of
objects.

BUG=None
TEST=Unit test

Change-Id: I477730dc91ed53a5d2e97a1f3c9065e81c703fee
Reviewed-on: https://chromium-review.googlesource.com/302700
Commit-Ready: Mao Huang <littlecvr@chromium.org>
Tested-by: Mao Huang <littlecvr@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
5 files changed