Fix asset resolver url handling

Summary:
1. file:// may get prepended to an http:// source URL during dev mode, making an invalid URL
2. the logic to detect `isLoadedFromFileSystem()` should've checked for file:// protocol to not get confused by http:// URL

Reviewed By: zahanm

Differential Revision: D6307187

fbshipit-source-id: e7e7a41bf721dd0601b0c1877e278e1e435ef5e2