init: replace Result<> with expected<>

Android-base has an implementation of the future std::expected<>.
This provides the same baseline functionality as Result<>, so use it
instead of our own version.

Bug: 132145659
Test: boot, init unit tests
Change-Id: I11e61bcb5719b262a6420483ed51a762826a9e23
diff --git a/init/service.cpp b/init/service.cpp
index ccc37b7..6887d7b 100644
--- a/init/service.cpp
+++ b/init/service.cpp
@@ -1383,7 +1383,7 @@
             continue;
         }
         if (auto result = service->Start(); !result) {
-            LOG(ERROR) << result.error_string();
+            LOG(ERROR) << result.error().as_string;
         }
     }
     delayed_service_names_.clear();