Not every Android contains the /system/framework directory, e.g. Brillo.
Test against other Android-only system files.

Fixes #14489.

Change-Id: I6d9ec1c4d4ceba3803798015e6917d59cf515de8
Reviewed-on: https://go-review.googlesource.com/19904Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Burcu Dogan <jbd@google.com>
Run-TryBot: Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>