Avoid memory leak in the Shutdown confirmation dialog.

How to reproduce:
1) Lock the screen.
2) Open the Phone options menu by long pressing the power
   button.
3) Tap "Power off" to display the confirmation dialog.
4) Repeat step 2 and 3 a few times (without closing the
   confirmation dialog.

Each time the confirmation dialog is displayed, a new
instance is created. A stack of confirmation dialogs are
created on the screen.

This is fixed by making sure the previous dialog is
dismissed before launching a new dialog.

Change-Id: I6b6c61ccc56364b66eed3528019f761e75bbe268