[Android] Require OpenGL ES 2.0

This was already an implicit requirement, as it's the only version implemented
in libwebrtc.

The reason to add this is to (defensively) try to filter old devices which may
not implement it.

WebRTC's own Android demo app defines this.

android/sdk/src/main/AndroidManifest.xml

     <uses-feature android:name="android.hardware.camera" />
     <uses-feature android:name="android.hardware.camera.autofocus"/>
 
+    <!-- Tell the system this app requires OpenGL ES 2.0. -->
+    <uses-feature android:glEsVersion="0x00020000" android:required="true" />
+
     <application android:allowBackup="true"
                  android:label="@string/app_name"
                  android:supportsRtl="true">