You asked, and we listened! As of the July 14 2015 update to Acrobat DC and Acrobat Reader DC, you can now hide the Tools panel on the right side of the Acrobat/Acrobat Reader user interface permanently.
For starters, please make sure that you've installed the July 14 2015 update by choosing Help > Check for Updates in Acrobat or Acrobat Reader.
There's nothing special that you need to do. Acrobat will remember the state of the Tools panel from one session to the next. Opened or closed, you choose! (You can toggle the Tools panel off by clicking the small arrow on its left border.)
Both Acrobat and Reader have been updated. But, there is no preference for Acrobat. You just hide the Tools panel by clicking the small arrow, and that setting will stick. The preference setting only applies to Acrobat Reader.