Add selinux labels for /metadata/ota.

/metadata/ota will store critical bits necessary to reify
system and vendor partition state during an OTA. It will be accessed
primarily by first-stage init, recovery/fastbootd, and update_engine.

Bug: 136678799
Test: manual test
Change-Id: Ib78cb96ac60ca11bb27d2b2fe011482e64ba0cf8
diff --git a/private/file_contexts b/private/file_contexts
index 5532bd3..aa2a42e 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -629,6 +629,7 @@
 /metadata/vold(/.*)?      u:object_r:vold_metadata_file:s0
 /metadata/gsi(/.*)?       u:object_r:gsi_metadata_file:s0
 /metadata/password_slots(/.*)?    u:object_r:password_slot_metadata_file:s0
+/metadata/ota(/.*)?       u:object_r:ota_metadata_file:s0
 
 #############################
 # asec containers