misc/cgo/testcshared: support testing on self-hosted android
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 <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,josharian/go,akutz/go,josharian/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,josharian/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,josharian/go,CAFxX/go,golang/go,nawawi/go,akutz/go,josharian/go,josharian/go,nawawi/go,golang/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go