bluetooth/file-transfer needs to be revised

bluetooth/file-transfer needs to be revised to fit Ubuntu 18.04
As Browse file is not long available since 16.04, bluetooth/file-transfer should be a combine of bluetooth pair and file transfer case.

The original bluetooth/browse-files and bluetooth/file-transfer are as follows

plugin: manual
category_id: com.canonical.plainbox::bluetooth
id: bluetooth/browse-files
depends: bluetooth/detect-output
estimated_duration: 120.0
_description:
 PURPOSE:
     This test will check that bluetooth connection works correctly
 STEPS:
     1. Enable bluetooth on any mobile device (PDA, smartphone, etc.)
     2. Click on the bluetooth icon in the menu bar
     3. Select 'Setup new device'
     4. Look for the device in the list and select it
     5. In the device write the PIN code automatically chosen by the wizard
     6. The device should pair with the computer
     7. Right-click on the bluetooth icon and select browse files
     8. Authorize the computer to browse the files in the device if needed
     9. You should be able to browse the files
 VERIFICATION:
     Did all the steps work?

plugin: manual
category_id: com.canonical.plainbox::bluetooth
id: bluetooth/file-transfer
depends: bluetooth/detect-output
estimated_duration: 120.0
_description:
 PURPOSE:
     This test will check that you can transfer information through a bluetooth connection
 STEPS:
     1. Make sure that you're able to browse the files in your mobile device
     2. Copy a file from the computer to the mobile device
     3. Copy a file from the mobile device to the computer
 VERIFICATION:
     Were all files copied correctly?

Updated bluetooth/file-transfer job should looks as follows

plugin: manual
category_id: com.canonical.plainbox::bluetooth
id: bluetooth/file-transfer
depends: bluetooth/detect-output
estimated_duration: 120.0
_description:
 PURPOSE:
     This test will check that you can transfer information through a bluetooth connection
 STEPS:
     1. Enable bluetooth on a target device (e.g. another laptop) and DUT (device under test)
     2. Launch bluetooth settings from menu bar
     3. Select target device from Device options for connection establishing
     4. Confirm PIN code on both DUT and target device
     5. Target device is connected
     7. Click target device in device list
     8. Click 'Send Files...' and choose a file to send in DUT

 VERIFICATION:
     Were file sent to target device ~/Downloads folder correctly?