Fix 5386408: Fix battery state information propagation in Lock Screen

This fixes a bug where Lock Screen would sometimes inappropriately show
"charged" if it took a while for Lock Screen to get an update on the
battery state.  It now starts with the state set to BATTERY_STATUS_UNKNOWN
so we properly update listeners when we finally get battery information
in handleBatteryUpdate().

Change-Id: I71018a233f38b2f897ff2e6592d7e310550fa016