Make setting the session token in MediaBrowserService synchronous

The MusicDemo was depending on the fact that setting the session token
is done synchronously and that getSessionToken can be called immediately
after. This makes setting the token synchronous again but leaves the
post to update any MediaBrowsers that are waiting on a connection.

This does introduce a small race condition if an app tries to set it
twice from different threads, but this is a very unlikely situation
and MediaBrowserService is not guaranteed to be thread safe.

bug:18603085
Change-Id: Id934c9f3b6520225cd92d1d8a6e2b3d606b8c6c8