Use String8/16 c_str [binder]

Current String8::string() has two problems: it may suggest it's
returning a std::string and also prevents a drop-in replacement
with std::string.

Bug: 295394788
Test: make checkbuild
Change-Id: I1eb6ddebe3faede57f3e6f046da572a79056125a
diff --git a/libs/fakeservicemanager/FakeServiceManager.cpp b/libs/fakeservicemanager/FakeServiceManager.cpp
index 80661c1..456e8ec 100644
--- a/libs/fakeservicemanager/FakeServiceManager.cpp
+++ b/libs/fakeservicemanager/FakeServiceManager.cpp
@@ -80,7 +80,7 @@
     for (const auto& [registeredName, service] : mNameToService) {
         (void) service;
         if (registeredName.startsWith(prefix)) {
-            out.add(String16(registeredName.string() + prefix.size()));
+            out.add(String16(registeredName.c_str() + prefix.size()));
         }
     }
     return out;