notify areaInfo enable/disable for all slots

Given areInfo enable/disable preference are singleton, we should notify
listners for all slots. Instead of using mAreaInfo, slot indexed map,
to retrieve slotIndex, simply loop through all available slots from
telephonyMgr API. VIVO BCCH 4G feature, which is currently not on
CellBroadcastService, will never be cleared when user sets preference
to false because CellBroadcastService mAreaInfos variable does not have
this information, thus it will never broadcast the enable/disable
status of the feature.

Bug: 162475765
Test: Manual
Change-Id: I2ed3f0c7643dfa8f5fccafa824d3566b900932c5