init: Make 'write_file' return bool to match 'read_file'.

The mismatch of return values makes reasoning about the correctness of
CLs like https://android-review.googlesource.com/317923 quite hard.

Bug: 33941660
Test: Init builds, HiKey boots.
Change-Id: Ia4b8a9af420682997b154a594892740181980921
diff --git a/init/util.h b/init/util.h
index e63c469..009413d 100644
--- a/init/util.h
+++ b/init/util.h
@@ -33,7 +33,7 @@
                   uid_t uid, gid_t gid, const char *socketcon);
 
 bool read_file(const char* path, std::string* content);
-int write_file(const char* path, const char* content);
+bool write_file(const char* path, const char* content);
 
 // A std::chrono clock based on CLOCK_BOOTTIME.
 class boot_clock {