Auto-detecting GOARM on Android makes as little sense as for nacl/arm
and darwin/arm.

Also update androidtest.sh to not require GOARM set.

Change-Id: Id409ce1573d3c668d00fa4b7e3562ad7ece6fef5
Reviewed-on: https://go-review.googlesource.com/98875Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>