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/Process.cpp

@@ -635,7 +635,7 @@ ErrorOr<Process::ScopedDescriptionAllocation> Process::OpenFileDescriptions::all
     return EMFILE;
 }
 
-Time kgettimeofday()
+Duration kgettimeofday()
 {
     return TimeManagement::now();
 }