docs: Clarified the SDK install instructions.

Noted that you need Android Studio 1.3 (canary) to test the M
Developer Preview, and added a section on how to install that.
Updated the "install the SDK" section to match AS 1.3's workflow.

Also removed reference to Nexus Player (per separate bug).

See first comment for doc stage location.

bug: 21422334
bug: 21425728
Change-Id: If8c62e074b883b4ba33753ac799b0a79739843b8