Remove static definition of writeStringToFile, and
move it from KeyStorage to Utils
(cherry picked from commit 0bd2d11692)
Bug: 71810347
Test: Build pass and reboot stress test.
Change-Id: I38bfd27370ac2372e446dc699f518122e73c6877
Merged-In: I38bfd27370ac2372e446dc699f518122e73c6877