RFR: 8133821: Refactor initialization of the heap and the collector policy

[Erik Helin]

Hi all,

this small patch refactors how the heap is set up to make the code a bit
more extensible.

Webrev:
http://cr.openjdk.java.net/~ehelin/8133821/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8133821

Testing:
JPRT

Thanks,
Erik