YaPay Java Fixes

[daxigua36]

• add Yandex Constants to paymentmethodtoken
• add contextInfo methods paymentmethodtoken to builders
• fix PaymentToken validation

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[daxigua36]

Hi everyone, is there anything I can do to get this merged?

[daxigua36]

@googlebot I signed it!

[juergw]

Sorry that we haven't replied earlier. paymentmethodtoken is not in active development, and we don't want to change/extend its API. So we are not going to merge this. Sorry.