init: restorecon libsnapshot rollback indicator

The rollback indicator is created during first stage init and
is unlabeled. Need to restorecon it properly so that it can
be properly deleted by update_engine later.

Bug: 149956852
Bug: 150907679
Test: apply OTA, reboot, look at serial console
Change-Id: Ie954023b557865691fec2ae378145596bf1973a9