Use String8/16 c_str [tools]

Bug: 295394788
Test: make checkbuild
Change-Id: I82d6899d8c15a10b15399c39177290012bb5f13b
diff --git a/tools/split-select/TestRules.cpp b/tools/split-select/TestRules.cpp
index 86ccd6a..ca3c56f 100644
--- a/tools/split-select/TestRules.cpp
+++ b/tools/split-select/TestRules.cpp
@@ -78,9 +78,8 @@
     const String8 actualStr(actual != NULL ? actual->toJson() : String8());
 
     if (expectedStr != actualStr) {
-        return ::testing::AssertionFailure()
-                << "Expected: " << expectedStr.string() << "\n"
-                << "  Actual: " << actualStr.string();
+        return ::testing::AssertionFailure() << "Expected: " << expectedStr.c_str() << "\n"
+                                             << "  Actual: " << actualStr.c_str();
     }
     return ::testing::AssertionSuccess();
 }