os: fix tests on brillo

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>

src/os/os_test.go

@@ -47,10 +47,10 @@ var sysdir = func() *sysDir {
 	switch runtime.GOOS {
 	case "android":
 		return &sysDir{
-			"/system/framework",
+			"/system/lib",
 			[]string{
-				"ext.jar",
-				"framework.jar",
+				"libmedia.so",
+				"libpowermanager.so",
 			},
 		}
 	case "darwin":