System routes are not automatically exposed

From this CL, even for system routes (phone speaker, bt routes),
features are required to get notified of events on routes or to
get them as "available" routes.

By adding this, apps can disable cast -> phone feature.

Bug: 159090706
Test: cts test && atest mediaroutertest && manually
using support v7 demos such that
  with LIVE_AUDIO : nothing changed
  w/o LIVE_AUDIO : cast -> phone feature is disabled
  unregistering callback : cast -> cast (media transfer) and
                           phone -> phone is only enabled <- this is
                           the expected behavior for apps that updates
                           AndroidX library

Change-Id: I4bd27eb1d4776b9cedb59b10e1bac5868d56d305