service: Refactor Client/Instance naming

Refactored the internal interfaces so that data structures with
per-application instances such as GattClient, GattServer, and
LowEnergyClient, and all other related data structures use "instance"
to generically refer to these instances rather than "client", which
was the naming inherited from BTIF.

Bug: 25650633
Change-Id: I1bda387f5d8fdccebe5eae7f058d7f8defafe8cc