Exclude org.json

Android provides it's own version of org.json, so it doesn't need to be bundled seperately.

Simply marking the dependency as provided (as was done earlier) doesn't help either since the dependency is also being added by the bugsnag-java dependency.

The right solution is to exclude the org.json dependency being provided by the bugsnag-java dependency, and instead implicitly depend on the one provided by Android.

Closes #16