Properly handle NowPlayingList Browsing commands

Properly handle NowPlayingList Browsing commands
by ensuring the following:
  - Change data type to store Max value of End index.
  - Change implementation to properly handle request
    for any un-supported attribute.
  - Properly store and use start index offset while
    responding with items in NowPlayingList.

CRs-Fixed: 753306 753307 753310

Change-Id: Iccce0ab0577277ebc5107c4e52ef1013eaeaeab3