Update transient navigation confirmation logic.

Show the confirmation toast when hiding the transient
navigation bar only the first time per app, per reboot.

Hitting the power key while the transient nav is hidden
is taken as a signal of possible user confusion, so reshow
the prompt the next time.

This requires the confirmation prompt to live in policy
(not sysui).  It arguably should have been here in the first
place, since the transient bar state/policy was here, and
sysui should not have not been able to fail to display
the prompt correctly.

Also take this opportunity to remove a hack wrt positioning
the confirmation properly while the nav bar is transitioning.
Toasts now support LAYOUT_HIDE_NAVIGATION if applied to the
toast's view.

Bug: 10246225
Change-Id: Ieb6355e4ca975c0758918a39e3c2ec13da81c7f4