Optimize keyguard/IME interactions

Change the keyguard window to LAYOUT_IN_SCREEN | LAYOUT_INSET_DECOR
and make the ViewManagerHost fitSystemWindows. This eliminates the
need to resize the actual window and associated surfaces when the IME
comes and goes.

Force the widget pager to measure at the fullscreen size of the
keyguard, even if the IME is showing. This causes the widgets to clip
instead of resize, removing a few more moving parts that can be
distracting/affect performance.

Partially improves bug 7427586

Change-Id: I0d86d0ca8045f737fa97ca5d1e7f6a49f746b999