Allow to subuscribe on getEffectiveBroadcast

:Release Notes:
Added possibility to subscribe on getEffectiveBroadcast

:Detailed Notes:

:Testing Performed:
Verified on desktop.

luna-send -f -i \
    luna://com.palm.systemservice/time/getEffectiveBroadcastTime \
    '{"subscribe":true}'

luna-send -f -n 1 \
    luna://com.palm.systemservice/time/setBroadcastTime \
    '{"utc":10,"local":30}'

:QA Notes:

:Issues Addressed:
[GF-41609] sys manager must provide a way to subscribe to Effective Broadcast Time

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Change-Id: Ie9eadd96c0537be29c63c3cb3ac1056b697e4966
Reviewed-on: https://g2g.palm.com/3627
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>