Fix bug in WriteStringToFileAtomic

According to https://www.slideshare.net/nan1nan1/eat-my-data ,
rename() without an fsync() is not safe, and cannot guarantee data
integrity in case of powerloss of OS failure.

Test: partner verification, th
Bug: 238702018
Change-Id: I5809770062ed7bfa47df81de418a2d8f7cbc6620