Store userspace reboot info in /metadata

Store pertinent information about userspace reboot events in the case
of failure. This information is any services which failed to stop
cleanly, the output of the default fstab and /proc/mounts, and
a list of mounts which failed to unmount. This information is only
stored as necessary (i.e. mount information will not be stored if
everything unmounted, even if some services failed to stop).

Added new /metadata/userspacereboot directory to persist this
information. Information older than 3 days will be deleted.

Test: adb reboot userspace with sigterm/sigkill timeouts set to
      very low values
Test: Manual test of storing all other information
Bug: 151820675
Change-Id: I6cfbfae92a7fc6f6c984475cad2c50c559924866