libsnapshot callstack files readable by dumpstate

Set callstack logs as 0644 so that they are readable
by dumpstate.

Test: take OTA, cancel, delete /data/misc/update_engine/prefs/*
manually, retrigger OTA (so that callstack is logged), then re-take bug report
Bug: 148818798

Change-Id: I598e484b57dcd3ce6ed7bb483aee57ce2ba881e7