Fix memory leak in plugins when navigating pages

- Cordova RTC work item number: 45147
- Apache Cordova Jira issue: https://issues.apache.org/jira/browse/CB-7028
- IBM PMR: 79005,999,000
- Full description: There was a memory leak when the network-information
  or device plugins were installed in a project that navigated between a
  native and hybrid page, due to failure to unwire events in
  NativeExecution.cs. (Changes reverted and re-added a safer fix)