csip: Initialize class field

'connecting_actively' was the field in GattServiceDevice which wasn't
set in the constructor or given a default value.  We give it a
default value to make it like the others and make any future bugs
where this isn't set before access easier to debug.

Bug: 150670922
Test: TreeHugger
Change-Id: I408cf435c490bed6d3785cf47a99369252763644