Token definitions fetching - better solution

[Nodonisko]

We should come up with better solution for fetching token definitions. We now have hardcoded only eth which is fine for now, but it's suboptimal.

Also it will fetch that definitions during startup which is also not good because parsing it is kind of slow. We should consider making it faster (react-native-quick-crypto) or prebundle these definitions (I like this one) and fetch them later just to be sure we have latest.