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/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