Additional safety regarding network activity indicator.

- Removed a redundant call to -decrementRequests for "single" requests. This
  would happen in the url completion block once the request is canceled. This
  was causing more calls to -decrementRequests than -incrementRequests.
- The request count is unsigned, so prevent underflow by checking for 0.
- Cancel pending delayed calls to hide the indicator when showing it.
Merge pull request #10 from walmartlabs-samsclub/issue/SCIPHONE-1582

Additional safety regarding network activity indicator.