[GATT Server] Expose IsolationManager from GATT server module

The IsolationManager is made a submodule of the GATT server, so we can
query it to figure out what services are exposed to each connection. It
is temporarily also available directly from FFI, solely for the purpose
of arbitration. This will be removed once the Rust server is used for
100% of GATT server operations.

Test: unit + CTS multi-device
Bug: 274945531
Change-Id: Idc3cdd78ec2435a7a286413d7b1ebaae83a8f07a