Can I have multiple working public keys per wallet? My Android Bitcoin wallet changes the Public key (the one I give to others) after every transaction for security.

However, I want to put up a Public key online for donations. 

Does this mean that my one wallet has multiple public keys (and I keep all the bitcoins given to any address owned by me) or is it that my private key changes also, and all BC given to my old key gets sent into the void?
Your wallet keeps all private and public keys it ever used indefinitely. You do not lose coins on old private keys when you give out new addresses.

It is just a privacy measure to give out new addresses for each transaction. It prevents your business partners from knowing about other transactions you have received and sent.

All addresses can be used multiple times, so you can use any of your addresses as a donation address. It will continue to work even when you give out other addresses later.

Be sure to keep a backup of your wallet separate from your phone if you are going to manage significant funds with it.