Explicitly mention again that startCommand() runs on the main thread.

This is already mentioned at the top of Service.java's javadoc, but
I'm seeing people get confused (myself included the other day) after
getting used to incoming service stub binder calls being on their own
threads and forgetting that startCommand() didn't work that way.

Change-Id: Ib79f48d9368ac0ba2d37b7e4dc98d53804bcd59e