misc/android: fix detection of GOROOT tests

strings.HasPrefix is not good enough to determine whether a path
is a subdirectory of another because it does not respect path
boundaries. filepath.Rel is good eonugh as long as we filter out results
that use parent directories, "..".

Hopefully fix the android emulator builders on the subrepositories.

Change-Id: I17ee7e0028c0b0b26a6c5f67629f53c9a660c6e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167117


Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>