add dummy GattServer implementation

it does not really register any service, but just got a basic
ui of fragment.
update wording
ui: use variant activity for chosen device

by default, Devices List always use Function Picker as its
target activity for chosen device.

Do not hard code it so Device List is more general.
ui: Add Activity Server List

to list supported Gatt Server for Bluebit
ui: Service List invoke Device Picker

it specify a Fragment to use in ActivityGattServer
rename: DevicesList -> DeviceChooser