[Standardize the Pay Button] Add support for new NVP shape private_lastPaymentMethod in App

[grgia]

Design Doc

Goal

Users with the old NVP shape will not run into issues when code references the new NVP shape.

Update all existing references in the app to check if private_lastPaymentMethod. is a string or an object to maintain backward compatibility for users with the old structure.

App should work regardless of which NVP shape the user has.

Context:

The previous private_lastPaymentMethod structure was:

private_lastPaymentMethod: {
   <policyID>: <preference>,
}

The updated structure in private_lastPaymentMethod will look like:

private_lastPaymentMethod: {
   <policyID>: {
		lastUsed: <preference>,
		Iou: <preference>,
		Expense: <preference>,
		Invoice: <preference>,
    }
}

[grgia]

@hungvu193 @getusha do you have enough context to take this one for App?

[getusha]

Yes, i think it's quite straightforward. I'll handle this one.

[hungvu193]

Draft PR is here: #56210

[hungvu193]

#56210 is ready for review!