Prevent premature display of starting window.

- When starting an activity, don't tell window manager to make the
windows visible is the keygaurd visibility for the activity is false.
Instead let the activity windows be made visible once the keygaurd
visibility transitions to true.
- Don't send duplicate visibility change request to window manger if we
are not changing state.

Fixes: 34545029
Test: Open a hangouts notification message on the lock-screen
and make sure the starting window doesn't flash.

Change-Id: I745e985766a1af97203e1d22b6443dabdd0c0363