Clarify constructTxParams parameters and deduplicate function

`constructTxParams` accepts an `amount` param and returns a tx object with a `value` param. This has long been a source of confusion. Also there are two `constructTxParams` implementations that can be reduced to one. This was originally solved in this draft PR https://github.com/MetaMask/metamask-extension/pull/9728 That became dated. We should reopen when it makes sense to prioritize

Start by locating both constructTxParams implementations and reviewing draft PR #9728 for the earlier approach. Trace callers to understand the amount/value naming and transaction shape; the work is done when one implementation remains, its parameter naming is clarified consistently, and affected tests pass.

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