vold: Introduce android::vold::writeStringToFile

Remove static definition of writeStringToFile, and
move it from KeyStorage to Utils

(cherry picked from commit 0bd2d116921ab46312cc4a37246a68d38447a72b)

Bug: 71810347
Test: Build pass and reboot stress test.
Change-Id: I38bfd27370ac2372e446dc699f518122e73c6877
Merged-In: I38bfd27370ac2372e446dc699f518122e73c6877
diff --git a/Utils.h b/Utils.h
index 48a57d9..c083021 100644
--- a/Utils.h
+++ b/Utils.h
@@ -134,6 +134,7 @@
 
 bool FsyncDirectory(const std::string& dirname);
 
+bool writeStringToFile(const std::string& payload, const std::string& filename);
 }  // namespace vold
 }  // namespace android