Make utility methods static.

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

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

tests/robotests/src/com/android/settings/fuelgauge/BatteryInfoTest.java

@@ -160,7 +160,7 @@ public class BatteryInfoTest {
                 .computeChargeTimeRemaining(anyLong());
         BatteryInfo info = BatteryInfo.getBatteryInfo(mContext, mChargingBatteryBroadcast,
                 mBatteryStats, SystemClock.elapsedRealtime() * 1000, false, 1000, false);
-        assertThat(info.remainingTimeUs = TEST_CHARGE_TIME_REMAINING);
+        assertThat(info.remainingTimeUs).isEqualTo(TEST_CHARGE_TIME_REMAINING);
         assertThat(info.remainingLabel.toString())
                 .isEqualTo(TEST_CHARGE_TIME_REMAINING_STRINGIFIED);
     }