Update Wifi Tether band selection UI

This CL updates the preference for selecting the enabled tether bands
to allow individual selection of multiple bands instead of forcing
a single selection. Note that while this makes the UI theoretically
support new bands in the future, the wifi backend only supports
selecting a single band or all bands which this CL does not address.

Test: robotests
Bug: 73102003
Change-Id: Ib2d4a2834c5cd11875515d308f7b20bfc8471959