startPreTransaction

Starts a pre-transaction wait ahead of the actual payment amount being known - triggers the background wait (see PreTransactionProgress class docs) on its own cancellable background thread right away, and returns a PreTransactionProgress immediately without waiting for it to complete. Call PreTransactionProgress.getResult to retrieve the outcome once available, or PreTransactionProgress.cancel to stop waiting.

Until the returned PreTransactionResult is continued via PreTransactionResult.startTransaction, startTransaction/startTransactionAsync refuse to run directly - see checkNoPendingPreTransaction.