Move all messaging constants to ExoCastConstants

Also make ExoCastMessage#mediaItemAsJsonObject protected.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=219780620

extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/ExoCastConstants.java

+/*
+ * Copyright (C) 2018 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.google.android.exoplayer2.ext.cast;
+
+/** Defines constants used by the Cast extension. */
+public final class ExoCastConstants {
+
+  private ExoCastConstants() {}
+
+  public static final int PROTOCOL_VERSION = 0;
+
+  // String representations.
+
+  public static final String STR_STATE_IDLE = "IDLE";
+  public static final String STR_STATE_BUFFERING = "BUFFERING";
+  public static final String STR_STATE_READY = "READY";
+  public static final String STR_STATE_ENDED = "ENDED";
+
+  public static final String STR_REPEAT_MODE_OFF = "REPEAT_MODE_OFF";
+  public static final String STR_REPEAT_MODE_ONE = "REPEAT_MODE_ONE";
+  public static final String STR_REPEAT_MODE_ALL = "REPEAT_MODE_ALL";
+
+  // Methods.
+
+  public static final String METHOD_BASE = "player.";
+
+  public static final String METHOD_ADD_ITEMS = METHOD_BASE + "addItems";
+  public static final String METHOD_MOVE_ITEM = METHOD_BASE + "moveItem";
+  public static final String METHOD_REMOVE_ITEMS = METHOD_BASE + "removeItems";
+  public static final String METHOD_SET_PLAY_WHEN_READY = METHOD_BASE + "setPlayWhenReady";
+  public static final String METHOD_SET_REPEAT_MODE = METHOD_BASE + "setRepeatMode";
+  public static final String METHOD_SET_SHUFFLE_MODE_ENABLED =
+      METHOD_BASE + "setShuffleModeEnabled";
+  public static final String METHOD_SEEK_TO = METHOD_BASE + "seekTo";
+  public static final String METHOD_SET_PLAYBACK_PARAMETERS = METHOD_BASE + "setPlaybackParameters";
+
+  // JSON message keys.
+
+  public static final String KEY_PROTOCOL_VERSION = "protocolVersion";
+  public static final String KEY_METHOD = "method";
+  public static final String KEY_SEQUENCE_NUMBER = "sequenceNumber";
+  public static final String KEY_INDEX = "index";
+  public static final String KEY_ITEMS = "items";
+  public static final String KEY_ARGS = "args";
+  public static final String KEY_UUID = "uuid";
+  public static final String KEY_UUIDS = "uuids";
+  public static final String KEY_LICENSE_SERVER = "licenseServer";
+  public static final String KEY_TITLE = "title";
+  public static final String KEY_URI = "uri";
+  public static final String KEY_REQUEST_HEADERS = "requestHeaders";
+  public static final String KEY_DESCRIPTION = "description";
+  public static final String KEY_MEDIA = "media";
+  public static final String KEY_DRM_SCHEMES = "drmSchemes";
+  public static final String KEY_START_POSITION_US = "startPositionUs";
+  public static final String KEY_END_POSITION_US = "endPositionUs";
+  public static final String KEY_MIME_TYPE = "mimeType";
+  public static final String KEY_PLAY_WHEN_READY = "playWhenReady";
+  public static final String KEY_REPEAT_MODE = "repeatMode";
+  public static final String KEY_SHUFFLE_MODE_ENABLED = "shuffleModeEnabled";
+  public static final String KEY_POSITION_MS = "positionMs";
+  public static final String KEY_SPEED = "speed";
+  public static final String KEY_PITCH = "pitch";
+  public static final String KEY_SKIP_SILENCE = "skipSilence";
+  public static final String KEY_PLAYBACK_STATE = "playbackState";
+  public static final String KEY_MEDIA_QUEUE = "mediaQueue";
+  public static final String KEY_IS_LOADING = "isLoading";
+  public static final String KEY_PLAYBACK_ERROR = "error";
+  public static final String KEY_PLAYBACK_POSITION = "playbackPosition";
+  public static final String KEY_PLAYBACK_PARAMETERS = "playbackParameters";
+}