CallTracker: Add back-off and retry toggle to getCurrentCalls

Add a build.prop variable to handle cases where the modem
reacts slowly and does not update CURRENT_CALLS to include calls which
are dialing. A delay is introduced to avoid spamming the modem and then
GetCurrentCalls is called again in order to avoid the calltracker from
marking the call as ended when it is still active.

Tested and LS970 and E970 (Sprint and ATT LG Optimus G).

Change-Id: I7d3aac00b811bceb2909adf239492962a737de4a