Use String8/16 c_str [media]

Bug: 295394788
Test: make checkbuild
Change-Id: I4ba5400f39c769bec59508525844f1daefe1d15c
diff --git a/media/libmedia/IMediaMetadataRetriever.cpp b/media/libmedia/IMediaMetadataRetriever.cpp
index 86427ed..cdb1837 100644
--- a/media/libmedia/IMediaMetadataRetriever.cpp
+++ b/media/libmedia/IMediaMetadataRetriever.cpp
@@ -218,7 +218,7 @@
             } else {
                 mMetadata.replaceValueFor(keyCode, value);
             }
-            return mMetadata.valueFor(keyCode).string();
+            return mMetadata.valueFor(keyCode).c_str();
         } else {
             return NULL;
         }