Add rules for calling ReadDefaultFstab()

Grant ReadDefaultFstab() callers
  allow scontext { metadata_file gsi_metadata_file_type }:dir search;
  allow scontext gsi_public_metadata_file:file r_file_perms;
so they can search / read DSU metadata files.
The DSU metadata files are required to deduce the correct fstab.

Also tighten the neverallow rules in gsid.te.

Bug: 181110285
Test: Build pass, presubmit test
Test: Boot and check avc denials
Test: Boot with DSU and check avc denials
Change-Id: Ie464b9a8f7a89f9cf8f4e217dad1322ba3ad0633
diff --git a/private/gsid.te b/private/gsid.te
index fb40528..e6a395a 100644
--- a/private/gsid.te
+++ b/private/gsid.te
@@ -166,8 +166,6 @@
     -init
     -gsid
     -fastbootd
-    -recovery
-    -vold
 } gsi_metadata_file_type:dir no_w_dir_perms;
 
 neverallow {
@@ -175,7 +173,6 @@
     -init
     -gsid
     -fastbootd
-    -vold
 } { gsi_metadata_file_type -gsi_public_metadata_file }:file_class_set *;
 
 neverallow {
@@ -183,7 +180,6 @@
     -init
     -gsid
     -fastbootd
-    -vold
 } gsi_public_metadata_file:file_class_set ~{ r_file_perms };
 
 # Prevent apps from accessing gsi_metadata_file_type.
@@ -193,15 +189,7 @@
     domain
     -init
     -gsid
-} gsi_data_file:dir *;
-
-neverallow {
-    domain
-    -init
-    -gsid
-    -fastbootd
-    -vold
-} gsi_data_file:file_class_set *;
+} gsi_data_file:dir_file_class_set *;
 
 neverallow {
     domain