Making using the PaymentMethodTokenRecipient less error prone.

You should now be able to just specify the public key manager when
contructing it instead of fetching the key yourself at construction
time.

This way if a developer ends up reusing the same recipient for multiple
messages the PaymentMethodTokenRecipient will take care of fetching
fresh verification public keys every single time.

Also clarifying in the docs what the typical expected usage should be.

Change-Id: Id7b5f88880e216f2a0624587284cf967b4e2353b
ORIGINAL_AUTHOR=Walter Cacau <waltercacau@google.com>
GitOrigin-RevId: 588337998e1525f4162124387449effe8f74a651