Add master audio balance setting

This adds an audio balance slider to accessibility allowing the end
user to adjust the audio balance between left and right channels.

Test: Change Balance through Settings, play audio
Bug: 28390736
Change-Id: I70f22d222ea310fd33ebe4f3fb054f44f69d2548

src/com/android/settings/accessibility/BalanceSeekBarPreference.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2019 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.accessibility;
+
+import android.content.Context;
+import android.media.AudioSystem;
+import android.os.UserHandle;
+import android.provider.Settings;
+import android.util.AttributeSet;
+import android.view.View;
+import android.widget.ImageView;
+import android.widget.SeekBar;
+
+import androidx.core.content.res.TypedArrayUtils;
+import androidx.preference.Preference;
+import androidx.preference.PreferenceViewHolder;
+
+import com.android.settings.R;
+import com.android.settings.widget.SeekBarPreference;
+
+/** A slider preference that directly controls audio balance **/
+public class BalanceSeekBarPreference extends SeekBarPreference {
+    private static final String TAG = "BalanceSeekBarPreference";
+    private final Context mContext;
+    private BalanceSeekBar mSeekBar;
+    private ImageView mIconView;
+
+    public BalanceSeekBarPreference(Context context, AttributeSet attrs) {
+        super(context, attrs, TypedArrayUtils.getAttr(context,
+                R.attr.preferenceStyle,
+                android.R.attr.preferenceStyle));
+        mContext = context;
+        setLayoutResource(R.layout.preference_balance_slider);
+    }
+
+    @Override
+    public void onBindViewHolder(PreferenceViewHolder view) {
+        super.onBindViewHolder(view);
+        mSeekBar = (BalanceSeekBar) view.findViewById(com.android.internal.R.id.seekbar);
+        mIconView = (ImageView) view.findViewById(com.android.internal.R.id.icon);
+        init();
+    }
+
+    private void init() {
+        if (mSeekBar == null) {
+            return;
+        }
+        final float balance = Settings.System.getFloatForUser(
+                mContext.getContentResolver(), Settings.System.MASTER_BALANCE,
+                0.f /* default */, UserHandle.USER_CURRENT);
+        // Rescale balance to range 0-200 centered at 100.
+        mSeekBar.setMax(200);
+        mSeekBar.setProgress((int)(balance * 100.f) + 100);
+        mSeekBar.setEnabled(isEnabled());
+    }
+}