RFR 8067447: Factor out the shared implementation of the VM flags manipulation code

[Jaroslav Bachorik]

Please, review the following change.

Issue : https://bugs.openjdk.java.net/browse/JDK-8067447
Webrev: http://cr.openjdk.java.net/~jbachorik/8067447/webrev.00
This patch is a precursor for implementing 
https://bugs.openjdk.java.net/browse/JDK-8054890 which itself is a part 
of JEP-228 (https://bugs.openjdk.java.net/browse/JDK-8043764).

Here, the code related to manipulating JVM flags is extracted to a 
separate ManagedFlags class and the codebease is adjusted to use this class.

Thanks,

-JB-