Use SystemStatusObserver to subscribe for dataitems in
XtraSystemStatusObserver.
Change-Id: Ib1828b9025c9c5bb5194a36014249472ed3f6f9e
CRs-Fixed: 2093290
Add DataItemObserver interface in SystemStatus so
that clients can subscribe dataitem events from
SystemStatus. Also moving DateItem and its observer,
subscription classes into HAL
Simplify DataItem class implementation.
Change-Id: I7b2c3fc9dcf67dd6132bbcccda874e3f7ed6fa94
CRs-Fixed: 2041019 2072896