healthd: break link between BatteryMonitor and BatteryPropertiesRegistrar

Make it easier to later separate out binder-related code.

Add helper functions healthd_battery_update(), healthd_get_property() to
allow these operations without needing references to the BatteryMonitor
object.

Change-Id: Ie584bf53e5178ce0a098d0d940d6c311fdff62d4
diff --git a/healthd/BatteryMonitor.cpp b/healthd/BatteryMonitor.cpp
index 5b3cac6..424d180 100644
--- a/healthd/BatteryMonitor.cpp
+++ b/healthd/BatteryMonitor.cpp
@@ -469,7 +469,7 @@
         KLOG_WARNING(LOG_TAG, "BatteryTechnologyPath not found\n");
 
     if (nosvcmgr == false) {
-            mBatteryPropertiesRegistrar = new BatteryPropertiesRegistrar(this);
+            mBatteryPropertiesRegistrar = new BatteryPropertiesRegistrar();
             mBatteryPropertiesRegistrar->publish();
     }
 }