Use String8/16 c_str [tools]

Bug: 295394788
Test: make checkbuild
Change-Id: I82d6899d8c15a10b15399c39177290012bb5f13b
diff --git a/tools/split-select/Grouper_test.cpp b/tools/split-select/Grouper_test.cpp
index 7294a86..a8b78cd 100644
--- a/tools/split-select/Grouper_test.cpp
+++ b/tools/split-select/Grouper_test.cpp
@@ -179,7 +179,7 @@
             errorMessage.append("\n");
         }
     }
-    ADD_FAILURE() << errorMessage.string();
+    ADD_FAILURE() << errorMessage.c_str();
 }
 
 void GrouperTest::addSplit(Vector<SplitDescription>& splits, const char* str) {