I integrated google anaylitics into my android game 5 days ago and published. Yesterday, I noticed that many sessions'(almost half of all sessions) duration is 00:00:00. Rest of sessions seems normal, no problem. My game has just 1 activity, it uses swipe tabs.
What is the main reason? How to solve?

You didn't include a link to your source code so all we can do is guess.

Generally missing session length values can happen when you miss a step. Double-check everything, starting here, then here, then here. Double-check that your application manifest is correct, that your resource files include the app_tracker.xml file properly configured, Also make sure your program actually sends events from time to time since duration is based on the time between the first event and the last event. Remember that putting code in finalizers isn't guaranteed to be called, so don't put your telemetry tracker code there. Make sure the DryRun flag is set to false.

Although it doesn't sound like the problem you are having, you should know that their telemetry is designed to lose data occasionally. They try to upload all the data but sometimes for various reasons they don't. Maybe their phone died, or cell signal went away, or wifi was turned off, whatever. Telemetry data is imperfect and is generally missing a chunk of data. It shouldn't be half of all sessions, though.