Support GKI updates

Adds proper file_contexts and domains for pre/postinstall hooks.
Allow the pre/postinstall hooks to communicate with update_engine stable
service.

Bug: 161563386
Test: apply a GKI update

Change-Id: I4437aab8e87ccbe55858150b95f67ec6e445ac1f
diff --git a/apex/Android.bp b/apex/Android.bp
index d3acfdb..78e1be9 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -77,6 +77,13 @@
 }
 
 filegroup {
+  name: "com.android.gki-file_contexts",
+  srcs: [
+    "com.android.gki-file_contexts",
+  ],
+}
+
+filegroup {
   name: "com.android.ipsec-file_contexts",
   srcs: [
     "com.android.ipsec-file_contexts",
diff --git a/apex/com.android.gki-file_contexts b/apex/com.android.gki-file_contexts
new file mode 100644
index 0000000..ccee7f8
--- /dev/null
+++ b/apex/com.android.gki-file_contexts
@@ -0,0 +1,2 @@
+(/.*)?       u:object_r:system_file:s0
+/bin/(.*)?   u:object_r:gki_apex_prepostinstall_exec:s0