Support clearing BrowserService session token

Applications may now call setSessionToken(null), which:
- Causes all connected browsers to disconnect.
- Allows the service to be terminated along with the
  hosting process.
- Allows the app to replace the browser's associated token
  once the session is released.

More details in go/invalid-mediabrowserservice-session.

Test: atest MediaBrowserTest MediaBrowserServiceTest
Bug: b/185136506
Change-Id: Idbbc23d735e905e948e5932ba8333ac79fd61b8e