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
4 files changed