Fix Android example

[ferjm]

Fixes #92

r? @Manishearth

[ferjm]

In case that you want to test this patch, you need to remove the code to switch backends based on the platform:

diff --git a/servo-media/Cargo.toml b/servo-media/Cargo.toml
index ab8ab27..cade577 100644
--- a/servo-media/Cargo.toml
+++ b/servo-media/Cargo.toml
@@ -12,5 +12,5 @@ path = "../audio"
 [dependencies.servo-media-player]
 path = "../player"

-[target.'cfg(all(not(target_os = "android"), target_arch = "x86"))'.dependencies.servo-media-gstreamer]
+[dependencies.servo-media-gstreamer]
 path = "../backends/gstreamer"
diff --git a/servo-media/src/lib.rs b/servo-media/src/lib.rs
index 91a1da6..65dddec 100644
--- a/servo-media/src/lib.rs
+++ b/servo-media/src/lib.rs
@@ -1,5 +1,4 @@
 pub extern crate servo_media_audio as audio;
-#[cfg(all(not(target_os = "android"), target_arch = "x86"))]
 extern crate servo_media_gstreamer;
 pub extern crate servo_media_player as player;
 use std::sync::{self, Arc, Mutex, Once};
@@ -37,10 +36,10 @@ impl PlayerBackend for DummyBackend {
     }
 }

-#[cfg(all(not(target_os = "android"), target_arch = "x86"))]
+//#[cfg(all(not(target_os = "android"), target_arch = "x86"))]
 pub type Backend = servo_media_gstreamer::GStreamerBackend;
-#[cfg(not(all(not(target_os = "android"), target_arch = "x86")))]
-pub type Backend = DummyBackend;
+//#[cfg(not(all(not(target_os = "android"), target_arch = "x86")))]
+//pub type Backend = DummyBackend;

 impl ServoMedia {
     pub fn new() -> Self