cherrypick from mnc-docs docs: Updated APK Signature Scheme v2 doc.

This version of the APK Signature Scheme v2 documentation doesn't
mention the "apksigner" command-line tool, since this feature isn't
supported as of I/O 2016. However, it will still be possible to sign
an app using APK Signature Scheme v2---and disable v2 signing---using
Android Studio 2.2 and Gradle.

This document is gated on the release of Android Studio 2.2,
scheduled to release on the first day of I/O (May 18).

Bug: 28143514
Change-Id: I00164a740bcc19b5ba08b517023cca11f4373680
Original-Change-Id: Ied916823424d38731e03073b07166f54e8a86b66