Add callbacks when TextClassifierService connection status change

Add onConnected() and onDisconnected() when TextClassifierService
connect status change. TextClassifierService can do initialization
in onConnected() and do cleanup work in onDisconnected(). These
callbacks also benefit tests to test some behavior after service
connected/disconnected.

Bug: 141111813
Test: Manual. Bind service and make sure onConnected() is called,
unbind service and make sure onDisconnected() is called.
Change-Id: Ic87f00e8fd6679ef8606feba728ad9db6ae6285e