os/exec: post-process lsof output on Android.

os/exec: post-process lsof output on Android.

lsof is used to inspect the open file desciptors in exec_test.go.
In order to limit the output of lsof to the tested process, the tests use
lsof with the -p option, but the version of lsof in android seems to ignore
it. This change adds a post-processing step to filter out irrelevant entries.

Fixes golang/go#10206.

Change-Id: Ia789b8f5e1e9b95c7b55deac92d0d1fbf3ee74fb
Reviewed-on: https://go-review.googlesource.com/8025
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>