Move the IME navigation guard view up to decor.

Although the IME windows are now allowed to extend into
the nav bar, some IMEs were making assumptions about
computed insets based on the height of the content view.

So our navigation bar view (opaque view blocking the nav bar
area to avoid the island effect when transparent) needs to live
above the content view in the hierarchy, making the content view
the same height as it was before.

A surgical spot to put the guard view is up at the root view
(PhoneWindow.DecorView).  fitSystemWindows is always called since
this view is not recreated, and the layout is stable: waiting until
the IME is attached to the window is too late to add a guard view.

This is above the screen_* layouts, so will work without having to
touch all of them.  And it only affects windows of TYPE_INPUT_METHOD.

Bug:11237795
Change-Id: I6a93f30aec83f1cecfb854073046cbc87ab4aa66