How to track multiple bitcoin addresses? Let's say I have multiple (N) wallets, with N public addresses and N private addresses. Is there a simple way to track the balance (and transactions, but this is not so important) of those wallets? Ideally I would install an android app with the capability of adding public addresses that allows me to see the balance of those addresses.

I do not need to be able to spend from those wallets, just to be able to see the balance. That is, I do not need to import the private addresses, just the public ones.
usually you can use block explorers to display amounts of your different bitcoin addresses. I tend to go to "blockchain.info". As it is cumbersome to cut and paste everytime, I created a small (Linux/MacOS/OpenBSD) shell script to do this for me (https://github.com/pebwindkraft/ccy_values). There are payed APIs and non payed APIs, the latter one having a limit on requests.