Simplify internal demo app

- Enable cleartext by default for the internal demo app
- Remove "external" variant of the internal demo app

PiperOrigin-RevId: 322987978

demos/main/src/main/AndroidManifest.xml

@@ -36,7 +36,7 @@
       android:allowBackup="false"
       android:allowBackup="false"
       android:requestLegacyExternalStorage="true"
       android:requestLegacyExternalStorage="true"
       android:name="androidx.multidex.MultiDexApplication"
       android:name="androidx.multidex.MultiDexApplication"
-      tools:ignore="UnusedAttribute">
+      tools:targetApi="29">
 
 
     <activity android:name="com.google.android.exoplayer2.demo.SampleChooserActivity"
     <activity android:name="com.google.android.exoplayer2.demo.SampleChooserActivity"
         android:configChanges="keyboardHidden"
         android:configChanges="keyboardHidden"