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/binder/MemoryDealer.cpp b/libs/binder/MemoryDealer.cpp
index 03553f3..5b1cb7e 100644
--- a/libs/binder/MemoryDealer.cpp
+++ b/libs/binder/MemoryDealer.cpp
@@ -428,7 +428,7 @@
 {
     String8 result;
     dump_l(result, what);
-    ALOGD("%s", result.string());
+    ALOGD("%s", result.c_str());
 }
 
 void SimpleBestFitAllocator::dump(String8& result,