Fix "Wrong password" message shown when using BT keyboard.

The code was assuming we're always using soft IME. When a physical
keyboard is attached, we get two events; ACTION_DOWN and ACTION_UP.
The code attempted to unlock the device twice.  The second attempt
failed because the TextView was cleared by the first successful attempt.

Fixes bug 17690585

Change-Id: Ia92a97400b4404d0cbe86aacb727009e95914327