Call onServiceDisconnected on callback executor

SharedConnectivityClientCallback.onServiceDisconnected is being called
directly on the main thread instead of the callback executor, which is
causing race conditions. Make sure to call this via the proxy so it runs
on the executor instead.

Flag: EXEMPT bugfix

Bug: 331652253
Test: atest SharedConnectivityManagerTest
Change-Id: I5f74632538fca10e0b6e2b4270345634b50d2513