Floss: Add UHID device creation

The UHID device is created when SLC is completed. It is destroyed when the
link with the headset disappear.

Send hook switch status to UHID on HF ATA or AT+CHUP

Manage UHID events by starting a blocking thread to read UHID output
events. This needs to duplicate the UHID file descriptor, and so the
uhid_virt crate is only used to generate or parse UHID events.

Bug: 277693919
Tag: #floss
Test: Conduct the following manual tests.
  $ btclient -c "telephony enable"
  pair Bluetooth Headset
  load https://google.git.io/libhidtelephoony/ with Chrome
  click on "Start", select the headset then "Connect"
  "Inputs Reports" and "Output Reports" are correct
  "Hook Switch", "Off-Hook" and "Ring" test cases are passed
Test: atest bluetooth_test_gd

Change-Id: Icae09ba0e3434816bbe3ad22f759b5c9685fc54c