Keyguard: Avoid unneeded SIM dialog show() calls

This commit makes the progress dialog's show() get called only once each time the SIM unlock process is started.

It was reported that a phone can be frozen by entering the PIN code for the SIM card "too quickly". This leaves the user with a device on which the screen is filled with a non-cancellable dialog box saying that the SIM card is being unlocked. The logs inform that Keyguard experienced a success and no explicit errors could be seen there and only the dialog stayed around, all leading to believe the issue is with how the progress dialog is being showed and later hidden (or simply dismissed), in which case this show() call reduction might be benefitial.

Change-Id: I5519dd30afcb043e4fc6d08748b296c93bb70589