Add MetadataRetriever demo.

Add a demo app that shows case how to use APIs from MetadataRetriever to
retriever track groups and frames from some media.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=207269221

demos/retriever/README.md

+# Exo Retriever demo application #
+
+This folder contains a demo application that showcases usage of ExoPlayer's
+Metadata and Frame Retrieval APIs.

demos/retriever/build.gradle

+// 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.
+apply from: '../../constants.gradle'
+apply plugin: 'com.android.application'
+
+android {
+    compileSdkVersion project.ext.compileSdkVersion
+    buildToolsVersion project.ext.buildToolsVersion
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    defaultConfig {
+        versionName project.ext.releaseVersion
+        versionCode project.ext.releaseVersionCode
+        minSdkVersion 16
+        targetSdkVersion project.ext.targetSdkVersion
+    }
+
+    buildTypes {
+        release {
+            shrinkResources true
+            minifyEnabled true
+            proguardFiles getDefaultProguardFile('proguard-android.txt')
+        }
+        debug {
+            jniDebuggable = true
+        }
+    }
+
+    lintOptions {
+        // The demo app does not have translations.
+        disable 'MissingTranslation'
+    }
+}
+
+dependencies {
+    implementation project(modulePrefix + 'library-core')
+    implementation project(modulePrefix + 'library-ui')
+    implementation project(modulePrefix + 'library-dash')
+    implementation project(modulePrefix + 'library-hls')
+    implementation project(modulePrefix + 'library-smoothstreaming')
+    implementation 'com.android.support:support-annotations:' + supportLibraryVersion
+}

settings.gradle

@@ -25,6 +25,7 @@ include modulePrefix + 'playbacktests'
 project(modulePrefix + 'demo').projectDir = new File(rootDir, 'demos/main')
 project(modulePrefix + 'demo-cast').projectDir = new File(rootDir, 'demos/cast')
 project(modulePrefix + 'demo-ima').projectDir = new File(rootDir, 'demos/ima')
+project(modulePrefix + 'demo-retriever').projectDir = new File(rootDir, 'demos/retriever')
 project(modulePrefix + 'playbacktests').projectDir = new File(rootDir, 'playbacktests')
 
 apply from: 'core_settings.gradle'