Fsync data after writing prefs

update_engine is relying on prefs stored on filesystem to resume OTA
progress. Partners report loss of update progress after forced reboot.
This might be I/O cache issue.

Test: th
Bug: 259174530
Change-Id: I7b4b8281cfe2cc45c51e4d6d60e09edb12f3ed86
diff --git a/common/utils.h b/common/utils.h
index 201e47e..0087794 100644
--- a/common/utils.h
+++ b/common/utils.h
@@ -162,6 +162,9 @@
 
 bool SendFile(int out_fd, int in_fd, size_t count);
 
+bool FsyncDirectory(const char* dirname);
+bool WriteStringToFileAtomic(const std::string& path, std::string_view content);
+
 // Returns true if the file exists for sure. Returns false if it doesn't exist,
 // or an error occurs.
 bool FileExists(const char* path);