Fix back button behavior and clean up in print preview activity.

The print preview UI owns and drives the lifecycle of several objects.
These objects were torn down too late resulting in an attempt to use
the activity as a context when it was no longer valid and service unbind
resulting in causing a crash.

Fixed a bug where if the back button is pressed mutiple times when
the print UI is being initialized a crash occurs.

bug:17454041

Change-Id: Ic1455b467586f0ad65f907e2160ec5cfb2d17d05