Allow custom keyguard "prompt reason"

Adds the possibility of presenting a custom message on
the keyboard if you're trying to dismiss the keyguard.

This is particularly useful when you're showing the
bouncer because custom authentication (SmarLock)
failed.

Test: launch FLAG_SHOW_WHEN_LOCKED activity and
      call KeyguardManager#requestDismissKeyguard
Fixes: 63940122
Change-Id: I0d88c0e59521887efa56d74874062b2b14970e4e