AK: Rename Time to Duration

That's what this class really is; in fact that's what the first line of
the comment says it is.

This commit does not rename the main files, since those will contain
other time-related classes in a little bit.

Kernel/Net/NetworkAdapter.cpp

@@ -95,7 +95,7 @@ void NetworkAdapter::did_receive(ReadonlyBytes payload)
         on_receive();
 }
 
-size_t NetworkAdapter::dequeue_packet(u8* buffer, size_t buffer_size, Time& packet_timestamp)
+size_t NetworkAdapter::dequeue_packet(u8* buffer, size_t buffer_size, Duration& packet_timestamp)
 {
     InterruptDisabler disabler;
     if (m_packet_queue.is_empty())