Add sepolicy for /metadata/watchdog

See go/rescue-party-reboot for more context.

One integer will be stored in a file in this
directory, which will be read and then deleted at the
next boot. No userdata is stored.

Test: Write and read from file from PackageWatchdog
Bug: 171951174

Change-Id: I18f59bd9ad324a0513b1184b2f4fe78c592640db
diff --git a/private/file_contexts b/private/file_contexts
index 633a6ce..d04fe92 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -744,6 +744,7 @@
 /metadata/bootstat(/.*)?  u:object_r:metadata_bootstat_file:s0
 /metadata/staged-install(/.*)?    u:object_r:staged_install_file:s0
 /metadata/userspacereboot(/.*)?    u:object_r:userspace_reboot_metadata_file:s0
+/metadata/watchdog(/.*)?    u:object_r:watchdog_metadata_file:s0
 
 #############################
 # asec containers