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

settings.gradle

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