Convert system/core to Result::ok()

No functionality changes, this is a mechanical cleanup.

Test: m
Test: cd system/core && atest
Change-Id: Ifdaa3ce1947ed578f656d5a446978726eb416c36
diff --git a/init/init_test.cpp b/init/init_test.cpp
index 9f63e4f..caf3e03 100644
--- a/init/init_test.cpp
+++ b/init/init_test.cpp
@@ -204,9 +204,9 @@
                                "execute 3";
     // clang-format on
     // WriteFile() ensures the right mode is set
-    ASSERT_TRUE(WriteFile(std::string(dir.path) + "/a.rc", dir_a_script));
+    ASSERT_RESULT_OK(WriteFile(std::string(dir.path) + "/a.rc", dir_a_script));
 
-    ASSERT_TRUE(WriteFile(std::string(dir.path) + "/b.rc", "on boot\nexecute 5"));
+    ASSERT_RESULT_OK(WriteFile(std::string(dir.path) + "/b.rc", "on boot\nexecute 5"));
 
     // clang-format off
     std::string start_script = "import " + std::string(first_import.path) + "\n"