Add /metadata/watchdog

This directory will be used to store the mitigation count
from Package Watchdog in the case of a boot loop, in
order to persist the value across fs-checkpointing
rollbacks. 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.

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

Test: Manual test using debug.crash_sysui property
      and inspecting file
Bug: 171951174
Change-Id: I2bd5e1ebe14d7e9e4f0e0dbeb90cf76b8400752e