Is it possible to hack an android device?

   Partially yes...

An android device can be hacked by using some tricks. There are lot more applications available in internet to spy on anyone's android device. Usually they are all paid applications and are undetectable in your devices you cant find them in your app lists or its functioning, but your personal credentials, pictures and lot more useful stuff can be viewed by hackers. 

Which device is going to be hacked is named as target device or victim's device. All information is transferred from the target device to hacker's device over the internet.

What they will do by spying on your devices?

    They spy on your device by inserting their Trojan files or any other malicious app into your phone. That app records all the activity which you done in that device. It acts a keylogger (records activity done by the keyboard), secretly camera is activated and it sends photos to the hacker's device, it can records the video even your device is locked. It sends all your call history, contact lists etc.. It also reads all the details regarding the device.

So attackers can do lot more with these information.

Somehow by improving the technology and strong security systems present android devices doesn't allows these malicious files installation. They shows an error message while installing this kind of apps into mobile phones. 

Mobile backdoor also a major technique to get into android device by using kali linux.