Allow entering PiP when pressing home while in car mode.

When system is in UI_MODE_TYPE_CAR pressing home button will relaunch
the driving mode overlay (aka gearhead/Morris Mini), which is a full
screen translucent activity. In that specific scenario if there is
an app that can enter PiP mode we should still allow that transition.

Bug: 276850167
Test: manually, in 3 button nav mode open Maps, start navigating,
swipe up to bring system nav bar and press home button there, Maps
will enter PiP mode.

Change-Id: Ic0037f81474e30e8c553c593dc73488072fe5a6e