Wallet: private listener lists as List type (was CopyOnWriteArrayList)
Wallet: private listener lists as List type (was CopyOnWriteArrayList)

This simplifies the declaration and verifies that we're using the standard List
interface in all our code. The implementation remains unchanged.

natzei/bitcoinj,bitcoinj/bitcoinj,natzei/bitcoinj,bitcoinj/bitcoinj