SystemUI: don't show recents if not provisioned

We don't show the recents button in the navbar if the device is not
provisioned, but the request can come from PhoneWindowManager by a
hardware button which has it mapped to recents. Respect the device
provisioned flag before allowing recents to be shown/toggled.

Ref: CYNGNOS-1070

Change-Id: I6e860192f1286ef48323c8aac33b5fd59c6f6e0c
Signed-off-by: Roman Birg <roman@cyngn.com>