vold: fsync both file and directory after write keys

Use vold version of writeStringToFile which fsync files, and
manually fsync directories after initialize global DE

(cherry picked from commit a98464f6)

Bug: 71810347
Test: Build pass and reboot stress test.
      Original boot failure symptom is NOT reproducible.
Change-Id: I1ca8f8cf0ccfd01075a9c33f79042e58d99aea26
Merged-In: I1ca8f8cf0ccfd01075a9c33f79042e58d99aea26