Summary:
This enables Fast Refresh by default.
More concretely:
* For clean installs, it's enabled by default. (You can opt out at any time via the dev menu.)
* For updated DEV apps (on device or simulator), it flips from disabled to enabled **but only if you never touched the setting before**.
* If you previously explicitly enabled and/or later disabled it, we keep your previous setting.
Reviewed By: yungsters
Differential Revision: D16442656
fbshipit-source-id: 1bfe0bf4bcf2830284f9c757fbfacc10db92acb4