Implement auto 12/24h setting toggle.

This implements an explicit toggle to enable/disable automatic 12h/24h
time formatting detection based on the current locale.

Previously automatic detection was the norm on a freshly wiped device,
but could never be re-enabled once either 12h or 24h format was
configured.

Bug: 32761619
Test: m RunSettingsRoboTests
Change-Id: Idbbb8f79fccec95e33bf2f12767d5736e1118fa7

src/com/android/settings/datetime/AutoTimeFormatPreferenceController.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright (C) 2017 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.datetime;
+
+import android.content.Context;
+import android.provider.Settings;
+import android.provider.Settings.System;
+import android.support.v14.preference.SwitchPreference;
+import android.support.v7.preference.Preference;
+import android.support.v7.preference.TwoStatePreference;
+import android.text.TextUtils;
+import android.text.format.DateFormat;
+import com.android.settings.core.PreferenceControllerMixin;
+import com.android.settingslib.core.AbstractPreferenceController;
+import java.util.Locale;
+
+public class AutoTimeFormatPreferenceController extends AbstractPreferenceController
+          implements PreferenceControllerMixin {
+
+    private static final String KEY_AUTO_24_HOUR = "auto_24hour";
+
+    public AutoTimeFormatPreferenceController(Context context, UpdateTimeAndDateCallback callback) {
+        super(context);
+    }
+
+    @Override
+    public boolean isAvailable() {
+        return true;
+    }
+
+    @Override
+    public String getPreferenceKey() {
+        return KEY_AUTO_24_HOUR;
+    }
+
+    @Override
+    public void updateState(Preference preference) {
+        if (!(preference instanceof SwitchPreference)) {
+            return;
+        }
+        ((SwitchPreference) preference).setChecked(isAutoTimeFormatSelection(mContext));
+    }
+
+    @Override
+    public boolean handlePreferenceTreeClick(Preference preference) {
+        if (!(preference instanceof TwoStatePreference)
+            || !TextUtils.equals(KEY_AUTO_24_HOUR, preference.getKey())) {
+            return false;
+        }
+        boolean auto24HourEnabled = ((SwitchPreference) preference).isChecked();
+        Boolean is24Hour;
+        if (auto24HourEnabled) {
+            is24Hour = null;
+        } else {
+            is24Hour = is24HourLocale(mContext.getResources().getConfiguration().locale);
+        }
+        TimeFormatPreferenceController.update24HourFormat(mContext, is24Hour);
+        return true;
+    }
+
+    boolean is24HourLocale(Locale locale) {
+        return DateFormat.is24HourLocale(locale);
+    }
+
+    /**
+     * Returns if the system is currently configured to pick the time format automatically based on
+     * the locale.
+     */
+    static boolean isAutoTimeFormatSelection(Context context) {
+        return Settings.System.getString(context.getContentResolver(), System.TIME_12_24) == null;
+    }
+}