When they say "bank", the term used in payments processing is "issuing bank", or more simply "issuer". The issuer is the entity that processes your application and issues you a card. They are responsible for approving purchases [0] on cards they issue and settling with the merchants.
So that list is basically the set of issuing banks which have done whatever steps are necessary to integrate with Android Pay.
[0] Typical flow looks something like this:
The merchant signs up with a payment processor who issues them a POS; this is the machine you swipe your card through. That swipe goes to the payment processor's network, which looks at the first 6 card digits, otherwise known as the Issuer Identification Number [1], and routes to the appropriate card network. For instance, any card starting with "37xxxx" is going to route to AmEx. The card network then routes to the issuing bank, who actually approves (or not) the transaction.
So POS -> Payment Processor -> Card -> Issuer.
[1] https://en.wikipedia.org/wiki/Bank_card_number#Issuer_identi...