Fix 5636798: clear PIN/PUK fields when device sleeps or user cancels

This bug is the result of onPause()/onResume() not being called properly
when we have both lock and unlock screens present, which is true for the SIM
and PUK unlock cases.

The fix is to invoke onPause() and onResume() on both of them if they exist.

Verified the rest of the code correctly handles having both.

Also, when the user cancels, we now clear the fields.

Change-Id: If7fbd31df7eab9e3d60656d50d2392607a4bf471