Bluetooth almost works.

Still to do:
1) Fix exception if bluetooth is turned off while link is up:
E/AndroidRuntime(13340): java.io.IOException: bt socket closed, read return: -1
E/AndroidRuntime(13340): 	at android.bluetooth.BluetoothSocket.read(BluetoothSocket.java:416)
E/AndroidRuntime(13340): 	at android.bluetooth.BluetoothInputStream.read(BluetoothInputStream.java:60)
E/AndroidRuntime(13340): 	at java.io.DataInputStream.readByte(DataInputStream.java:96)
E/AndroidRuntime(13340): 	at org.mavlink.MAVLinkReader.readNextMessage(MAVLinkReader.java:179)
E/AndroidRuntime(13340): 	at org.mavlink.MAVLinkReader.getNextMessage(MAVLinkReader.java:130)
E/AndroidRuntime(13340): 	at com.geeksville.mavlink.MavlinkStream$$anonfun$com$geeksville$mavlink$MavlinkStream$$rxWorker$2.apply(MavlinkStream.scala:90)
E/AndroidRuntime(13340): 	at com.geeksville.mavlink.MavlinkStream$$anonfun$com$geeksville$mavlink$MavlinkStream$$rxWorker$2.apply(MavlinkStream.scala:74)
E/AndroidRuntime(13340): 	at com.geeksville.util.Using$.using(Using.scala:20)
E/AndroidRuntime(13340): 	at com.geeksville.mavlink.MavlinkStream.com$geeksville$mavlink$MavlinkStream$$rxWorker(MavlinkStream.scala:74)
E/AndroidRuntime(13340): 	at com.geeksville.mavlink.MavlinkStream$$anonfun$1.apply(MavlinkStream.scala:37)
E/AndroidRuntime(13340): 	at com.geeksville.mavlink.MavlinkStream$$anonfun$1.apply(MavlinkStream.scala:37)
E/AndroidRuntime(13340): 	at com.geeksville.util.ThreadTools$$anon$1.run(ThreadTools.scala:11)
E/AndroidRuntime(13340): 	at java.lang.Thread.run(Thread.java:856)

2) Fix repeated parameter fetching