[GATT Server] Simplify IsolationManager interface

Some methods were redundant and existed only for FFI. Now that the
module is decoupled from FFI, I'm pushing the complexity to the
call-site and simplifying the core API.

Test: unit + CTS multi-device
Bug: 274945531
Change-Id: I1e5591b01a2edca37904baf7f7e186d7cc84ee38