RFR: 8237261: Concurrent refinement activation threshold not updated for card counts

[Kim Barrett]

Please review this change to the activation threshold for the primary
(first) concurrent refinement thread.  The special calculation used
for that thread's threshold wasn't updated to account for the change
from using buffer counts to using counts of the cards in the buffers
by JDK-8230109.

Also fixed a parameter name that wasn't updated by that same change
from buffer counts to card counts.

CR:
https://bugs.openjdk.java.net/browse/JDK-8237261

Webrev:
https://cr.openjdk.java.net/~kbarrett/8237261/open.00/

Testing:
mach5 tier1 by itself
mach5 tier1-5 and some perf testing with in development change for JDK-8237143