[1.9] Make AutoConfigurator more resilient to unknown physical memory size

1. Makes figuring out the physicalMemory in the AutoConfigurator more resilient to ClassLoader restrictions that would prevent loading the interface that has the getTotalPhysicalMemorySize method.
2. Makes AutoConfigurator handle being unable to detect the physical memory behave in a better way, by still trusting the heap size and not treating it as if the heap has been configured too large.