I suggest that the back button should work more similar to other chat applications on the mobile platform. At the moment pressing the back button (or gesture) prompts for confirmation to close the application.
It feels more natural to go back to the previous channel, direct message, etc.

For example:

I receive a direct message notification and I click on it. I am taken to the direct message. I then activate the drawer and move to another channel. To get back to the prevoius direct message or channel it would be handy to just press the back key.

Similarly I would press the back key until there are no more screens in history at which time pressing back again should prompt to exit the application.