How to re-run the "app optimization" process after upgrading to a higher version of Android? After each OTA Android upgrade, I get a message saying "Optimizing app x of xxx" that takes a while. The last time I upgraded, the battery ran out during the optimization process (not during the upgrade itself, which I know it's dangerous). After rebooting the phone the "optimization" process didn't carry on.

So, how can I start that process by itself without upgrading anything? (my Android is already up to date).
Go into the recovery (you need to boot the phone while pressing several buttons - you'll need to google which one is set for your device - an example would be Volume down + Home) and delete the "Dalvik cache". Then restart your phone. Now the optimizing process should start again.

But I personally recommend to not do that - it can't hurt, but if your phone works well you don't need this, because it's just like a Backup of your app installation files.