Load selinux compat file if it exists

This addresses Treble backwards compat issues introduced in
aosp/793958 and aosp/783669.

Bug: 122874820
Test: build/flash blueline with pi-dev vendor and generic_ab system
    images.
Test: adb pull /sys/fs/selinux/policy;
    sesearch policy --allowx -s vendordomain -t dev_type

Change-Id: I9123aa02cf4bf07d72d1ea086dde0d1cf1b30db4
diff --git a/init/selinux.cpp b/init/selinux.cpp
index 132fc13..c49dc9f 100644
--- a/init/selinux.cpp
+++ b/init/selinux.cpp
@@ -331,6 +331,12 @@
     }
     std::string plat_mapping_file("/system/etc/selinux/mapping/" + vend_plat_vers + ".cil");
 
+    std::string plat_compat_cil_file("/system/etc/selinux/mapping/" + vend_plat_vers +
+                                     ".compat.cil");
+    if (access(plat_compat_cil_file.c_str(), F_OK) == -1) {
+        plat_compat_cil_file.clear();
+    }
+
     std::string product_policy_cil_file("/product/etc/selinux/product_sepolicy.cil");
     if (access(product_policy_cil_file.c_str(), F_OK) == -1) {
         product_policy_cil_file.clear();
@@ -376,6 +382,9 @@
     };
     // clang-format on
 
+    if (!plat_compat_cil_file.empty()) {
+        compile_args.push_back(plat_compat_cil_file.c_str());
+    }
     if (!product_policy_cil_file.empty()) {
         compile_args.push_back(product_policy_cil_file.c_str());
     }