RFR: JDK-8240476: SystemPropertiesWriter does not conform to standard page layout

[Jonathan Gibbons]

Please review a simple localized fix to construct the System Properties page
using the recently-new standard flex-box layout.

For reference, the modified code was based on equivalent code in 
SingleIndexWriter.generateIndexFile.

The existing test for the page is extended with additional checks for 
the new behavior.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8240476
Webrev: http://cr.openjdk.java.net/~jjg/8240476/webrev.00/index.html