Relabel /data/rollback files as enabling rollback (4/n)

We will link files under /data/apex/active and /data/app to
/data/rollback when enabling rollbacks to avoid copy.

When creating hard links, we have to enusre source and target have the
same label to avoid subtle bugs.

We will assign apk_data_file to *.apk files and
staging_data_file to *.apex files under /data/rollback.

Also allow system_server to link /data/apex/active files.

Bug: 168562373
Test: m
Change-Id: I4be38cc8c84494c4ddfa03e37f2af3958bff5dfb
diff --git a/private/system_server.te b/private/system_server.te
index 90061c6..b60740b 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -519,6 +519,9 @@
 allow system_server staging_data_file:dir create_dir_perms;
 allow system_server staging_data_file:file create_file_perms;
 
+# Manage /data/rollback.
+allow system_server staging_data_file:{ file lnk_file } { create_file_perms link };
+
 # Walk /data/data subdirectories.
 allow system_server app_data_file_type:dir { getattr read search };