Limit external/slf4j visibility.

SLF4J is used in mockftpserver only and both of them are used in tests
only.
FTP support probably could be dropped, but in mean time we can
explicitly point out that none of SLF4J or mockftpserver reaches prod
code / env.

Bug: 210532587
Bug: 210594275
Test: atest FtpURLConnectionTest
Change-Id: Ic1257ee5ec44dc093e017ca55fc462666d735892
diff --git a/Android.bp b/Android.bp
index 5b9941a..e653f3f 100644
--- a/Android.bp
+++ b/Android.bp
@@ -65,4 +65,9 @@
     ],
 
     sdk_version: "core_current",
+    // SLF4J is used by mockftpserver, both of them are used in tests only.
+    visibility: [
+        "//external/mockftpserver",
+        "//libcore",
+    ],
 }