Make USB feature optional

Label the "USB host" feature as optional, otherwise devices without
USB host won't be listed as compatible.

android/AndroidManifest.xml

@@ -7,7 +7,8 @@
   <uses-sdk android:minSdkVersion="9" 
       android:targetSdkVersion="17"
       /> <!-- 9 = Gingerbread -->
-  <uses-feature android:name="android.hardware.usb.host" />
+  <uses-feature android:name="android.hardware.usb.host"
+      android:required="false"/>
   <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
   <application android:icon="@drawable/icon" android:label="@string/app_name">
     <activity