docs: Fixed targetSdkVersion in Auto Backup code snippets

Changed targetSdkVerion from "android-MNC" to "MNC". Also--per
preview/setup-sdk.html -- changed the minSdkVersion to "MNC" in
both places (if people are building apps for the preview release,
the minimum SDK version has to be MNC to keep the app from getting
installed onto devices running production Android).

See first comment for doc stage location.

bug: 21443485
Change-Id: I4cc5f16d53cd795b2db156c99db7c46f14d0ce65