handle "opaque" xmpp: URIs from QR scan

unfortunately, Uri.getQueryParameter() only works with "hierarchical" URIs.

This fixes this crash:
java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=49374, result=-1, data=Intent { act=com.google.zxing.client.android.SCAN flg=0x80000 (has extras) }} to activity {info.guardianproject.otr.app.im/info.guardianproject.otr.app.im.app.NewChatActivity}: java.lang.UnsupportedOperationException: This isn't a hierarchical URI.
at android.app.ActivityThread.deliverResults(ActivityThread.java:3432)
at android.app.ActivityThread.handleSendResult(ActivityThread.java:3475)
at android.app.ActivityThread.access$1300(ActivityThread.java:139)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1258)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5086)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.UnsupportedOperationException: This isn't a hierarchical URI.
at android.net.Uri.getQueryParameter(Uri.java:1662)
at info.guardianproject.otr.app.im.app.NewChatActivity.onActivityResult(NewChatActivity.java:1349)
at android.app.Activity.dispatchActivityResult(Activity.java:5446)
at android.app.ActivityThread.deliverResults(ActivityThread.java:3428)
... 11 more