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/action.cpp b/init/action.cpp
index 94ccef2..a4f6936 100644
--- a/init/action.cpp
+++ b/init/action.cpp
@@ -127,7 +127,7 @@
     // report such failures unless we're running at the DEBUG log level.
     bool report_failure = !result.has_value();
     if (report_failure && android::base::GetMinimumLogSeverity() > android::base::DEBUG &&
-        result.error_errno() == ENOENT) {
+        result.error().as_errno == ENOENT) {
         report_failure = false;
     }
 
@@ -139,7 +139,7 @@
 
         LOG(INFO) << "Command '" << cmd_str << "' action=" << trigger_name << " (" << filename_
                   << ":" << command.line() << ") took " << duration.count() << "ms and "
-                  << (result ? "succeeded" : "failed: " + result.error_string());
+                  << (result ? "succeeded" : "failed: " + result.error().as_string);
     }
 }