Use String8/16 c_str [tools]

Bug: 295394788
Test: make checkbuild
Change-Id: I82d6899d8c15a10b15399c39177290012bb5f13b
diff --git a/tools/split-select/Rule_test.cpp b/tools/split-select/Rule_test.cpp
index c6cff0d..c78533f 100644
--- a/tools/split-select/Rule_test.cpp
+++ b/tools/split-select/Rule_test.cpp
@@ -68,7 +68,7 @@
     expected.erase(std::remove_if(expected.begin(), expected.end(), ::isspace), expected.end());
 
     // Result
-    std::string result(rule.toJson().string());
+    std::string result(rule.toJson().c_str());
     result.erase(std::remove_if(result.begin(), result.end(), ::isspace), result.end());
 
     ASSERT_EQ(expected, result);