Use String8/16 c_str

Bug: 295394788
Test: make checkbuild
Change-Id: I538720d4de2dcc038b417048209782ff605b7e30
diff --git a/media/jni/android_media_ImageWriter.cpp b/media/jni/android_media_ImageWriter.cpp
index 2c498e5..dd1652c9 100644
--- a/media/jni/android_media_ImageWriter.cpp
+++ b/media/jni/android_media_ImageWriter.cpp
@@ -1068,7 +1068,7 @@
         String8 msg;
         msg.appendFormat("Format 0x%x is opaque, thus not writable, the number of planes (%d)"
                 " must be 0", writerFormat, numPlanes);
-        jniThrowException(env, "java/lang/IllegalArgumentException", msg.string());
+        jniThrowException(env, "java/lang/IllegalArgumentException", msg.c_str());
         return NULL;
     }