BLE: code for fetching and parsing battery level (not used).

Part of #24.

src/main/java/de/dennisguse/opentracks/data/models/BatteryLevel.java

@@ -0,0 +1,18 @@
+package de.dennisguse.opentracks.data.models;
+
+public class BatteryLevel {
+
+    public static BatteryLevel of(int percentage) {
+        return new BatteryLevel(percentage);
+    }
+
+    private final int percentage;
+
+    private BatteryLevel(int percentage) {
+        this.percentage = percentage;
+    }
+
+    public int getPercentage() {
+        return percentage;
+    }
+}

src/main/java/de/dennisguse/opentracks/sensors/BluetoothUtils.java

@@ -28,6 +28,7 @@
 import java.util.List;
 import java.util.UUID;
 
+import de.dennisguse.opentracks.data.models.BatteryLevel;
 import de.dennisguse.opentracks.data.models.Cadence;
 import de.dennisguse.opentracks.data.models.Distance;
 import de.dennisguse.opentracks.data.models.HeartRate;
@@ -48,6 +49,11 @@ public class BluetoothUtils {
 
     public static final UUID CLIENT_CHARACTERISTIC_CONFIG_UUID = new UUID(0x290200001000L, 0x800000805f9b34fbL);
 
+    public static final ServiceMeasurementUUID BATTERY = new ServiceMeasurementUUID(
+            new UUID(0x180F00001000L, 0x800000805f9b34fbL),
+            new UUID(0x2A1900001000L, 0x800000805f9b34fbL)
+    );
+
     public static final ServiceMeasurementUUID HEARTRATE = new ServiceMeasurementUUID(
             new UUID(0x180D00001000L, 0x800000805f9b34fbL),
             new UUID(0x2A3700001000L, 0x800000805f9b34fbL)
@@ -102,6 +108,17 @@ public static boolean hasBluetooth(Context context) {
         return BluetoothUtils.getAdapter(context) != null;
     }
 
+    public static BatteryLevel parseBatteryLevel(BluetoothGattCharacteristic characteristic) {
+        // DOCUMENTATION org.bluetooth.characteristic.battery_level.xml
+        byte[] raw = characteristic.getValue();
+        if (raw.length == 0) {
+            return null;
+        }
+
+        final int batteryLevel = characteristic.getIntValue(BluetoothGattCharacteristic.FORMAT_UINT8, 0);
+        return BatteryLevel.of(batteryLevel);
+    }
+
     public static HeartRate parseHeartRate(BluetoothGattCharacteristic characteristic) {
         //DOCUMENTATION https://www.bluetooth.com/wp-content/uploads/Sitecore-Media-Library/Gatt/Xml/Characteristics/org.bluetooth.characteristic.heart_rate_measurement.xml
         byte[] raw = characteristic.getValue();