beta testing: handle wallet birthday height for non-mainnet chain ids

[TalDerei]

from @grod220: "noticed with the beta testers they were not receiving funds when they go through the new seedphrase flow in onboarding for testnet. But then realized that walletCreationBlockHeight is before the rpc selection screen and derived from the rpcs in the registry (all mainnet). That means their wallet creation height was higher than the latest block height on testnet, and therefore were skipping block syncing. Think we should do two things:

• On rpc selection screen, if we see they choose an rpc with a non-mainnet chain id, we should delete or recalculate walletCreationBlockHeight
• On advanced -> clear cache, we should delete walletCreationBlockHeight in storage (or accept an option to delete creation height in local storage)"

[grod220]

Along with this work: we should use the fetched registry and not the bundled one (only as a backup if fetch fails)