Add offerId to purchase method in SubscriptionsManager

[nalcalag]

Task/Issue URL: https://app.asana.com/0/1201807753394693/1209226459318682/f

Description

Include offerId argument in subscriptions purchase method

Steps to test this PR

Pre steps

• Apply patch to being able to test subscriptions in staging

Privacy Pro Eligible US

• Make sure you are US eligible
• Install form branch
• Go to Settings
• Check you can see Privacy Pro option
• Tap on that option
• Check Subscriptions page looks as expected with full price subscriptions
• Purchase any of the options
• Check Free Trial is not offered when GooglePlay dialog for purchase is shown
• Subscribe
• Check everything works as expected

Privacy Pro Eligible ROW (Optional)

• Make sure you are ROW eligible
• Install form branch
• Check browser works as expected
• Go to Settings
• Check you can see Privacy Pro option
• Tap on that option
• Check Subscription page looks as expected
• Purchase any of the options and check it works well
• Go to Subscription Settings
• Check everything works as expected

No UI changes

[nalcalag]

• Add offerId to purchase method in SubscriptionsManager #5532 👈 (View in Graphite)
• develop

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[lmac012]

LGTM, works as expected.

[lmac012]

nit: I doesn't seem like we need that default value here. Right now, SubscriptionManager is always passing a value for offerId. Without that default value it will be harder to miss that parameter e.g., when refactoring something in the future.