Convert metricUnits boolean into an enum.

Fixes #1199.

src/main/java/de/dennisguse/opentracks/settings/UnitSystem.java

@@ -0,0 +1,23 @@
+package de.dennisguse.opentracks.settings;
+
+import de.dennisguse.opentracks.R;
+
+public enum UnitSystem {
+    METRIC(R.string.stats_units_metric),
+    IMPERIAL(R.string.stats_units_imperial);
+
+    private final int preferenceIdentifier;
+
+    UnitSystem(int preferenceIdentifier) {
+        this.preferenceIdentifier = preferenceIdentifier;
+    }
+
+    public int getPreferenceId() {
+        return preferenceIdentifier;
+    }
+
+    @Deprecated //TODO used to initialize before loading from preferences; should be loaded first
+    public static UnitSystem defaultUnitSystem() {
+        return METRIC;
+    }
+}