Add system captions manager service

This service connects to a remote system captions manager service. This
service is responsible for enabling system captions when the user
requests them. As the system binds to it, this service will be
persistent.

Cherry pick from ag/6761232

Bug: 128925852
Test: Manual. I created an implementation of the service.
Merged-In: Iafde1bb68f4754d8167624f47c6833d43c0ec336
Change-Id: Iafde1bb68f4754d8167624f47c6833d43c0ec336