all: add GOOS=android

As android and linux have significant overlap, and
because build tags are a poor way to represent an
OS target, this CL introduces an exception into
go/build: linux is treated as a synonym for android
when matching files.

http://golang.org/s/go14android
https://groups.google.com/forum/#!topic/golang-dev/P1ATVp1mun0

LGTM=rsc, minux
R=golang-codereviews, mikioh.mikioh, dave, aram, minux, gobot, rsc, aram.h, elias.naur, iant
CC=golang-codereviews, rsc
https://golang.org/cl/105270043