Update Android implementation, remove Android gateway validation, refactor canMakePayments

[riso]

This MR changes a few things in the current Android implementation.

• Update to non-deprecated gradle directives
• Updates to use AndroidX
• Updates Android implementation to use current, non-deprecated Google APIs
• Remove validation of gateways for Android. Google adds/changes supported gateways and new gateways work transparently without having to do any change. No reason to reject unknown gateways arbitrarily from the library

This also refactors canMakePayments. This is a breaking change, and changes how canMakePayments is exposed. Is no longer part of PaymentRequest, but it's directly exposed as a function. While having canMakePayments as part of PaymentRequest makes the API more compliant with the specification, there's no point in having to create a PaymentRequest just to verify that the device can make payments. For both Apple and Google Pay in fact, we only need to supply a small subset of the payment data (nothing at all in case of Apple Pay) to verify that the device supports the payment methods.

[ChrisFromCanada]

I'm pretty sure the GooglePay API update is a requirement as of the end of this month. How do we get someone to merge it in?!

[dswiconnect]

Nice job! This PR should be merged, otherwise this lib doesn't make any sense @naoufal @Andreyco

[nabilfreeman]

Hey there, thanks for doing this work. I'm helping out with this library. The PR has been open for a while and I'm not quite the expert on the Android side of things and Google Pay. What's the situation as of late 2020 with this?

[litinskii]

+1 for merge this pr