Get list of active media

:Release Notes:
1) Implement handler for request 'listActiveMedia'
2) Add request for active shards and convert to json

:Detailed Notes:

:Testing Performed:
tested onbuild

:QA Notes:
luna-send -n 1 luna://com.palm.db/listActiveMedia '{}'
response (no active shards):
{"returnValue":true,"count":0,"media":[]}

:Issues Addressed:
[GF-11591] DB8 Ext: Add shard logic to LS2 method for active media

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <ikuksyuk@luxoft.com>

Change-Id: Iaca4dbd86f4afef38b086685e444483307bdc61f
Reviewed-on: https://g2g.palm.com/3148
Reviewed-by: Build Verification
Reviewed-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Tested-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>