We'd like the ability to add Flutter to existing iOS applications
without requiring that they set `FLTLibraryPath` and `FLTAssetsPath`
in the main bundle's `Info.plist`.

1. Modify `-[FlutterDartProject initWithPrecompiledDartBundle:]` to
   support setting the library and assets path from the specified
   `NSBundle` instead.

2. If no `NSBundle` is explicitly specified, look for one with a
   default bundle identifier ("io.flutter.flutter.app") before
   falling back to the main NSBundle.

Also remove `+[FlutterDartProject pathForFlutterAssetsFromBundle:]`
because we don't use it internally, and it isn't exposed in the
header file.