Add log for BatteryUtils

This cl add logs to get more data for app usage time.

Also refactor BatteryUtils to singleton pattern because we need
to extract package name from uid.

Bug: 36909166
Test: RunSettingsRoboTests
Change-Id: I6ede354035c46e46cb856b7cf57bd1b7ccbd6d4f

tests/robotests/src/com/android/settings/applications/InstalledAppDetailsTest.java

@@ -74,7 +74,7 @@ public final class InstalledAppDetailsTest {
     ApplicationFeatureProvider mApplicationFeatureProvider;
     @Mock(answer = Answers.RETURNS_DEEP_STUBS)
     private UserManager mUserManager;
-    @Mock
+    @Mock(answer = Answers.RETURNS_DEEP_STUBS)
     private SettingsActivity mActivity;
     @Mock
     private DevicePolicyManager mDevicePolicyManager;