Check if getSystemSessionInfo requested by a Proxy Router

ProxyRouter (inside MediaRouter2) calls getSystemSessionInfoForPackage
to request the initial RoutingSessionInfo. The method was designed for
LocalRouter and does not take into account calls from proxy routers,
therefore returns DEFAULT_ROUTE when requested by system router but
without bluetooth or modify audio routing permissions.

Bug: b/325755852
Test: N/A
Change-Id: I9fa38fb731c17e06927e1a927d6acc030ebb2135