Add Stat to finder/fs

Add a Stat method to finder/fs that will be used by finder to read
the mode of the target of a symlink.

Bug: 157656545
Test: fs_test.go
Change-Id: Ie2b4509b7d11857d9a1685de4477088b91d43c63
diff --git a/finder/fs/Android.bp b/finder/fs/Android.bp
index 835eb0b..b71934f 100644
--- a/finder/fs/Android.bp
+++ b/finder/fs/Android.bp
@@ -25,6 +25,7 @@
         "test.go",
     ],
     testSrcs: [
+        "fs_test.go",
         "readdir_test.go",
     ],
     darwin: {