Migrate String8/16 from .string() to c_str()

This will align usage with std::string.

Bug: 295394788
Test: make checkbuild
Change-Id: Ic5c215b011197950dcbcb0339f44cc68fefc65a1
diff --git a/libutils/RefBase.cpp b/libutils/RefBase.cpp
index ab122c7..e0a2846 100644
--- a/libutils/RefBase.cpp
+++ b/libutils/RefBase.cpp
@@ -330,7 +330,7 @@
                      this);
             int rc = open(name, O_RDWR | O_CREAT | O_APPEND, 0644);
             if (rc >= 0) {
-                (void)write(rc, text.string(), text.length());
+                (void)write(rc, text.c_str(), text.length());
                 close(rc);
                 ALOGI("STACK TRACE for %p saved in %s", this, name);
             }