Clone System properties before iterating

Iterating over the live set retrieved from System.getProperties() can
cause trouble as any modification to the System properties during
iteration here will trigger concurrent modification exception. It's
safer to first clone the live set and iterate over the clone.

Bug: 36820084
Test: 40000 iterations of CTS unit tests
Change-Id: I647a6cc8ab9d3a51204e635bfd3250c035a77e68