Encrypted Media: Confine UUID code to MediaDrmBridge.

Currenly we have UUID in a lot of places in the EME implementation on Chromium
for Android. But UUID should really just be an implementation detail of
MediaDrmBridge. This CL moves all UUID related code to MediaDrmBridge and uses
key system elsewhere.

BUG=347596
R=boliu@chromium.org, dcheng@chromium.org, ddorwin@chromium.org, joi@chromium.org
TBR=dcheng@chromium.org

Review URL: https://codereview.chromium.org/185993004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255727 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255727