Fix a bug in the handling of recent_activity
The problem occurred when the idle GC was turned off with +RTS -I0. Then the scheduler would go into the state ACTIVITY_DONE_GC directly without doing a GC, and a subsequent GC would put it back to ACTIVITY_YES but without turning the timer back on. Instead if the GC finds the state is ACTIVITY_DONE_GC it should leave it there.
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