UserDataPreparer: reboot to recovery for system user only

With the next CL, old devices might contain a combination of old users
with prepareUserStorage error checking disabled and new users with
prepareUserStorage error checking enabled.  Factory resetting the whole
device when any user fails to prepare may be too aggressive.  Also,
UserDataPreparer already destroys the affected user's storage when it
fails to prepare, which seems to be fairly effective at breaking things
for that user (absent proper error handling by upper layers).

Therefore, let's only factory reset the device if the failing user is
the system user.

Bug: 164488924
Bug: 216025819
Change-Id: Ia1db01ab4ec6b3b17d725f391c3500d92aa00f97