Make utility methods static.

BatteryUtils.convertUsToMs and .convertMsToUs should be static, and now
they are.

Bug: 63347148
Test: make RunSettingsRoboTests
Change-Id: If652e2d3e1260df9a933805d7da670fbb26b2c25

src/com/android/settings/fuelgauge/BatteryUtils.java

@@ -320,11 +320,11 @@ public class BatteryUtils {
         }
     }
 
-    public long convertUsToMs(long timeUs) {
+    public static long convertUsToMs(long timeUs) {
         return timeUs / 1000;
     }
 
-    public long convertMsToUs(long timeMs) {
+    public static long convertMsToUs(long timeMs) {
         return timeMs * 1000;
     }