service: Add new GattServer abstraction

Added bluetooth::GattServer and bluetooth::GattServerFactory,
which conform the BluetoothClientInterface and
BluetoothClientInterfaceFactory interfaces. A GattServer represents
a per-application server instance that was registered with the stack
(a GATT server API "client").

The new code was added to the gatt_server.h|.cpp files, replacing
the old code that was added for cast. The old code has been moved to
the gatt_server_old.h|.cpp files.

Bug: 24245347
Change-Id: I3ca2eb53ab8320c9dba656859a4a89683607bb22