Changes to support navigation bar system UI in car mode.

The change has all the platform changes required to support
modifications in the navbar dimensions and custom icons in car mode.
The UX is not frozen yet, but have placeholder resources provided
by android auto UX engineers.
The change assumes that the car mode configuration is known to the
WindowManagerService and uses its current ui mode to request the
latest from the policy (PhoneWindowManager.java). The change is
modeled on the way rotation is handled, where the Policy knows the
different view attributes for uiMode and just returns back the
window sizes based on the current uiMode requested. The policy does
know the current uiMode, but the order of when that changes is not
deterministic [from logs it does happen before any request to update
UI occurs, but guess that could change].

Bug: 25996809
Change-Id: Ia46cbe5096382d26c9eb8ec74cf59a059b767edb