Follow-up to Google Pay integration

[dbrant]

A couple of minor enhancements:

• Per feedback in the previous PR:

Do we need to update the preference logic to make sure the user clicks one of the options in the GPay bottomsheet instead of saving the preference when tapping on the the dialog "Donate now"?

This is now fixed: the campaign will be considered "shown" only when the user clicks one of the buttons in the DonateDialog, instead of the primary button in CampaignDialog.

• The "maximum" donation amount is received from the API, but is only defined for USD. To apply the maximum amount for other currencies, we need to calculate the maximum based on the proportion of the minimum USD to the minimum of the target currency.