Ensure IME windows always appear north of the nav bar.

Even if the bars are transparent / invisible.

Refactor the PSB bar mode override logic to take the IME hint into
account.  Bars are overriden to opaque when hint is showing.

Initialize bar state to showing, fix bug that would mark state
as hidden pre-show (on shell restart).

Bug:10505132
Bug:9499956
Change-Id: I21e830e90c7e9812b4192ca65c3c0cd7a6b72798