snapshotctl logs readable by dumpstate

Set persistent logs for snapshotctl to 0644
so that they are readable by dumpstate.

Not using mode field in open() because it is masked
by umask. Directly use fchmod instead.

Test: reboot and take bugreport
Bug: 148818798

Change-Id: I515f8fd1345fcfb82aa2a1ec0c95da4b6921c039