Use SystemUI to show reboot UI on TV

Before, the system presented a mobile-themed progress dialog
when rebooting.
By setting `config_showSysuiShutdown` to `true`, the
reboot UI is presented by SystemUI instead.
SystemUI's current implementation shows a full-screen scrim overlay,
with centered text and spinner, feeling more at home on TV.

Bug: 127762211
Test: Settings -> Reboot
Change-Id: Ieb7b8157fd0a06a167e1d7545439672691864ff1