doc: Updated branding guidelines per request from @deniseamling.

Per her team's request, I also removed the AI version of the Google
Play Store icon, and replaced it with a high-rez PNG version.

Current version of doc is staged at:

http://asolovay.mtv:9010/distribute/tools/promote/brand.html

(Denise gave me a LGTM via email.)

Bug: 14621461

Change-Id: Id44a0dc73c7ef2f2bde595a1e98b75083d126176