[a11y] Set focus on "Pairing device" text once QR code is scanned.

Bug: 152462657

Test: Manual. Enable TalkBack in Settings > Accessibility. Navigate to
Developer options > Wireless debugging > Pairing device with QR code.
Scan a valid QR code, which should change to "Pairing device..." and
screen reader should say "Pairing device".

Change-Id: If467b56f787bda765fce1e5d46f8ab3c6210dc76