Set fvisibility=hidden per default.

This reduces the size of libtraced_shared.so by ~half.

Bug: 71844955
Change-Id: Ie961920297af9dc08287cc25c815d46c8ae266f3
diff --git a/Android.bp b/Android.bp
index d487b4a..eb9d198 100644
--- a/Android.bp
+++ b/Android.bp
@@ -116,6 +116,7 @@
     "-Wno-sign-compare",
     "-Wno-sign-promo",
     "-Wno-unused-parameter",
+    "-fvisibility=hidden",
   ],
   local_include_dirs: [
     "include",