misc/cgo/testcshared: support testing on self-hosted android

Only invoke adb for android if we're not running on android already.

Change-Id: I4eb94286a5bf09b382716a0474f3aebec40f5d74
Reviewed-on: https://go-review.googlesource.com/c/go/+/170953


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>