deps: Upgrade React Native to v0.68

This is the main upgrade commit for #5344.  Some follow-up steps
remain to be done in the next few commits.

In the template app, this corresponds to:

 * Numerous commits just bumping the `react-native` dependency

 * One commit bumping Metro:
     facebook/react-native@ea74c5797 Bump direct Metro dependencies to 0.67.0

 * One commit updating AppDelegate.m:
     facebook/react-native@aef843bfe update jsBundleURLForBundleRoot in template project
   to correspond to an API change in RN itself:
     facebook/react-native@0912ee179 remove fallbackResource from RCTBundleURLProvider api

Other changes:

 * Update Podfile.lock after `yarn install`.

 * We get to remove a Flow fixme, because it was (as hoped for)
   fixed in RN upstream:
     facebook/react-native@851e87a1a make Easing an object, not a class

Author: gnprice