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/subcontext_benchmark.cpp b/init/subcontext_benchmark.cpp
index eae03e3..6307993 100644
--- a/init/subcontext_benchmark.cpp
+++ b/init/subcontext_benchmark.cpp
@@ -39,7 +39,7 @@
     free(context);
 
     while (state.KeepRunning()) {
-        subcontext.Execute(std::vector<std::string>{"return_success"}).IgnoreError();
+        subcontext.Execute(std::vector<std::string>{"return_success"});
     }
 
     if (subcontext.pid() > 0) {