More userActivity() calls in keyguard

Some security screens aren't currently calling userActivity().  As such,
they allow keyguard to timeout before the user has a chance to enter
the required information.

The fix uses a TextWatcher to look for changes in the input text
and call userActivity() appropriately.

bug 7291431

Change-Id: I6d7889cc01a4d6bdbefefc5af478e812c35b1a49