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/IActivityManager.cpp b/libs/binder/IActivityManager.cpp
index 08169f5..5ec4e8b 100644
--- a/libs/binder/IActivityManager.cpp
+++ b/libs/binder/IActivityManager.cpp
@@ -52,8 +52,8 @@
                 }
             } else {
                 // An exception was thrown back; fall through to return failure
-                ALOGD("openContentUri(%s) caught exception %d\n",
-                        String8(stringUri).string(), exceptionCode);
+                ALOGD("openContentUri(%s) caught exception %d\n", String8(stringUri).c_str(),
+                      exceptionCode);
             }
         }
         return fd;