Re-label UART read/write characteristics

[jessegreenberg]

I am pretty sure the UI labels for UART are reversed. I think the following commit is correct, but wanted to confirm with @brettfiedler . Now we have

'micro:bit UART RX (write)'
'micro:bit UART TX (read)'

I think this matches notes from https://lancaster-university.github.io/microbit-docs/resources/bluetooth/bluetooth_profile.html

For RX - This characteristic allows a connected client to send a byte array (meaning paper-playground would write)
For TX - "This characteristic allows the micro:bit to transmit" (meaning paper-playground would read)

[jessegreenberg]

This is going to be verified by #246, closing