binder: Implement |OnStateChanged| callback

Add state change callbacks. This is used by clients to monitor wpa_supplicant's
connection status, etc.

BUG: 30093041
TEST: Ran a simple connect/disconnect integration test gtest.

Change-Id: I35238f0f95b0a0a4723cb315ddaba30dd19d7dab
Signed-off-by: Roshan Pius <rpius@google.com>
6 files changed