healthd: Add "No UI" operation for charger mode

Boards can opt to have charger mode be passive, with no screen UI or key
processing, the same as for recovery mode, via BOARD_CHARGER_NO_UI.  The
minui and associated libs and charging image files are not built into the
executable in this case.

Always select charger No UI operation for Brillo devices.

Bug: 27170356
Change-Id: Ibe71498743353b01ce5b6521e42ed94d75547547