PreferenceDrawerActivity: Customize back button behavior

Patch Set 1:
* This will open the drawer on the first back press.
* Terminate the app on second back press
* Added an handler to reset the variable after 5 secs

Patch Set 2:
* Make it acting like Google Hangouts:
If the drawer is open -> Finish
If the drawer is closed -> Open it

Patch Set 3 and 4:

* Removed the Handler and the double back as it's no more needed, this is the perfect behavior!
* Little derp

Change-Id: I5ce4430b3e3c6dcd03198634b62b4dc18dca604b
Signed-off-by: iGio90 <giovanni.rocca.90@gmail.com>