The `metamaskNetworkId` property in the `txMeta` for incoming
transactions was incorrectly set as a Number instead of a String. This
change was made accidentally as part of #8627.
As a result incoming transactions were being excluded from the
transaction list, as they didn't have a matching network ID.
`metamaskNetworkId` is now set to a string, and a migration has been
added to ensure `metamaskNetworkId` is converted to a string for any
incoming transactions in state.
