mediaserver: wait before instantiating services

- Check with service manager if it holds a reference to the
  service.
- If it does, repeat the same check after a second.
- Timeout after five such unsuccessful attempts.
- This "wait and proceed" is needed to ensure that services
  get registered with the service manager in the event mediaserver
  gets re-spawned after it got killed.

Change-Id: I65cf988959f57bc8bddec6322704b5f6307fc276
CRs-Fixed: 282339