MediaSessionManager: Apply API council review

Here's the comments from the review
  - RemoteUserInfo needs to override .hashCode() as well
  - Verify package name and the UID in the RemoteUserInfo
  - Docs: Document how to get RemoteUserInfo
  - Docs: Document that RemoteUserInfo is only valid on thread that the
          command is being called from
    * It's already documented in the
      MediaSession#getCurrentControllerInfo() and
      MediaBrowserService#getCurrentBrowserInfo().

Bug: 77507337
Test: Run following CtsMediaTestCases
  - MediaBrowserTest
  - MediaBrowserServiceTest
  - MediaControllerTest
Change-Id: If37560a7bc59d1ef5517ec1f3e0a8b74c2bb2980