sharedmem.cpp: replace dup() with fcntl(F_DUPFD_CLOEXEC)

Replace calls to dup() with fcntl(F_DUPFD_CLOEXEC). The only difference
between the two is that O_CLOEXEC is set on the newly duped file
descriptor. This helps address file descriptor leaks crossing an exec()
boundary.

Test: compiles and boots
Change-Id: I95218331382b7a3432d31a46c173009dc4df1a6b

native/android/sharedmem.cpp

@@ -71,7 +71,7 @@ int ASharedMemory_dupFromJava(JNIEnv* env, jobject javaSharedMemory) {
     }
     int fd = env->CallIntMethod(javaSharedMemory, sSharedMemory.getFd);
     if (fd != -1) {
-        fd = dup(fd);
+        fd = fcntl(fd, F_DUPFD_CLOEXEC, 0);
     }
     return fd;
 }