docs: Revised page to include info about new Publisher API.

Also moved the blurb to the "Google Play Distribution" cover page, and
adjusted the TOC accordingly. Per DDoughterty@ 's request, moved the
"Purchase Verification Strategies" section to billing_subscriptions.jd

Page is staged to:

http://asolovay.mtv:9543/google/play/billing/gp-purchase-status-api.html
http://asolovay.mtv:9543/google/play/billing/billing_subscriptions.html#strategies

Change-Id: I884e2f3f25c76fcb055e663058d1a4f4642fb558