Create the demos dir

Now the demo app is under demos/main.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163967883

settings.gradle

@@ -20,7 +20,7 @@ if (gradle.ext.has('exoplayerModulePrefix')) {
 
 include modulePrefix + 'demo'
 include modulePrefix + 'playbacktests'
-project(modulePrefix + 'demo').projectDir = new File(rootDir, 'demo')
+project(modulePrefix + 'demo').projectDir = new File(rootDir, 'demos/main')
 project(modulePrefix + 'playbacktests').projectDir = new File(rootDir, 'playbacktests')
 
 apply from: 'core_settings.gradle'