init: replace Result<Success> with Result<void>

Now that Result<T> is actually expected<T, ...>, and the expected
proposal states expected<void, ...> as the way to indicate an expected
object that returns either successfully with no object or an error,
let's move init's Result<Success> to the preferred Result<void>.

Bug: 132145659
Test: boot, init unit tests
Change-Id: Ib2f98396d8e6e274f95a496fcdfd8341f77585ee
diff --git a/init/util.cpp b/init/util.cpp
index 243e5f0..14acaa2 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -197,7 +197,7 @@
     return rc;
 }
 
-Result<Success> WriteFile(const std::string& path, const std::string& content) {
+Result<void> WriteFile(const std::string& path, const std::string& content) {
     android::base::unique_fd fd(TEMP_FAILURE_RETRY(
         OpenFile(path, O_WRONLY | O_CREAT | O_NOFOLLOW | O_TRUNC | O_CLOEXEC, 0600)));
     if (fd == -1) {
@@ -206,7 +206,7 @@
     if (!android::base::WriteStringToFd(content, fd)) {
         return ErrnoError() << "Unable to write file contents";
     }
-    return Success();
+    return {};
 }
 
 bool mkdir_recursive(const std::string& path, mode_t mode) {