Set defaults in RCTBundleURLProvider so development mode is the default

[jsierles]

This PR follows up from #8400 and 9845f49.

Sets URL parameters for ensuring dev mode is enabled by default.

Test Plan

Install React Native in a fresh app from react-native init.

update AppDelegate.m to use the new default development URL generator:

jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index.ios" fallbackResource:nil];

Run on iOS device or simulator, and ensure that the dev menu can be pulled up.

[ghost]

By analyzing the blame information on this pull request, we identified @nathanajah and @shaneosullivan to be potential reviewers.

[javache]

@nathanajah is looking into changing this so we apply the defaults in the constructor.

We should also get rid of the dispatch_once.

[jsierles]

@javache Sounds good - would be good to get this into 0.29 to avoid confusion on the issue:

#8333 (comment)

[ghost]

It's been a while since the last commit was reviewed and the labels show this pull request needs review. Based on the blame information for the files in this pull request we identified @nathanajah as a potential reviewer. Could you take a look please or cc someone with more context?

[javache]

I think this has been fixed already, right?