Impl highlight effect on BatteryChartView slots.

Screenshot: https://screenshot.googleplex.com/F5VrGjj5kdNHMV6

Bug: 284893240
Bug: 291689623
Test: manual
Change-Id: I846d95d31e8bb839481b86a94d5191ff205f8328

src/com/android/settings/fuelgauge/batteryusage/BatteryChartViewModel.java

@@ -55,6 +55,7 @@ class BatteryChartViewModel {
     private final String[] mFullTexts;
 
     private int mSelectedIndex = SELECTED_INDEX_ALL;
+    private int mHighlightSlotIndex = SELECTED_INDEX_INVALID;
 
     BatteryChartViewModel(@NonNull List<Integer> levels, @NonNull List<Long> timestamps,
             @NonNull AxisLabelPosition axisLabelPosition,
@@ -106,6 +107,14 @@ class BatteryChartViewModel {
         mSelectedIndex = index;
     }
 
+    public int getHighlightSlotIndex() {
+        return mHighlightSlotIndex;
+    }
+
+    public void setHighlightSlotIndex(int index) {
+        mHighlightSlotIndex = index;
+    }
+
     @Override
     public int hashCode() {
         return Objects.hash(mLevels, mTimestamps, mSelectedIndex, mAxisLabelPosition);