Don't depend on String8 cast to C string

Bug: 295394788
Test: m checkbuild
Change-Id: I6aa039b6b2a4944e3537ef133f8785890d957edd
diff --git a/media/jni/android_media_MediaDescrambler.cpp b/media/jni/android_media_MediaDescrambler.cpp
index c61365a..37111c2 100644
--- a/media/jni/android_media_MediaDescrambler.cpp
+++ b/media/jni/android_media_MediaDescrambler.cpp
@@ -368,7 +368,7 @@
 
     ScopedLocalRef<jstring> msgObj(
             env, env->NewStringUTF(msg != NULL ?
-                    msg : String8::format("Error %#x", serviceSpecificError)));
+                    msg : String8::format("Error %#x", serviceSpecificError).c_str()));
 
     return (jthrowable)env->NewObject(
             clazz.get(), ctor, serviceSpecificError, msgObj.get());