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.

MERGED from commit 396f0903