Only update UI when bound

If the PrintActivity gets destroyed (e.g. due to low memory) it gets
re-created when the PrintToPDF activity returns in onActivityResult. In
this case onActivityResult is called right after onCreate and before the
UI is bound. This resulted in a NullPointerException when accessing the
unbound UI elements.

Bug: 21749058
Change-Id: I44e625e8c4f10432591d0afcf7825d7e0dbee6f5