The chosen token in the `send` flow was set from one of two places:
`metamask.selectedTokenAddress` or `metamask.send.token`. The former is
used most of the time, but the latter is used for the 'Edit' button
shown in the upper-left of the confirmation UI.
The send flow will now exclusively use `metamask.send.token` for the
token state during the send flow. `metamask.selectedTokenAddress` is
now only used for the selected token state on the Home screen. This
simplifies the Redux state, as the send token is now in one place
instead of two, and `metamask.selectedTokenAddress` has only one
purpose.
