Add a method to calculate battery usage diff results from history

Bug: 184807417
Test: make SettingsRoboTests
Test: make SettingsGoogleRoboTests
Change-Id: If613c5cfd7e00e1883107385d3d8552774378cd7

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

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.settings.fuelgauge;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+
+@RunWith(RobolectricTestRunner.class)
+public final class BatteryDiffEntryTest {
+
+    @Test
+    public void testSetTotalConsumePower_returnExpectedResult() {
+        final BatteryDiffEntry entry =
+            new BatteryDiffEntry(
+                /*foregroundUsageTimeInMs=*/ 10001L,
+                /*backgroundUsageTimeInMs=*/ 20002L,
+                /*consumePower=*/ 22.0,
+                /*batteryHistEntry=*/ null);
+        entry.setTotalConsumePower(100.0);
+
+        assertThat(entry.getPercentOfTotal()).isEqualTo(22.0);
+    }
+}