Remove rolled out flag enableNullSessionInMediaBrowserService

This flag is always true now.

Also update the Javadoc to reflect that passing a null value is
allowed.

Bug: 409604982
Flag: com.android.media.flags.enable_null_session_in_media_browser_service
Test: existing tests
Change-Id: Iab62d7d1c0d3df4ea80bdb766a86ff22e3311716