Robustness: wrap any calls to System.getProperty in case security is enabled.

Since calling System.getProperty may throw an exception on locked-down systems.

Note: this patch has the side-effect of making the 'guice_include_stack_traces'
setting static, rather than querying the system property each time a module is
installed. Making the setting static improves performance as System.getProperty
can be a bottleneck, at the cost of being able to change the setting on-the-fly.

If the 'guice_include_stack_traces' setting should be dynamic then I can amend
the patch accordingly.
WDYT of moving this into InternalFlags and abstracting the method into something like:
```
  static String getProperty(String name, String defaultValue) {
    try {
      AccessController.doPrivileged(() -> return System.getProperty(name);
    } catch(SecurityException se) {
      return defaultValue;
    }
  }
```
(with the lambda expanded)