[a11y] Fix TalkBack readback on adb qrcode fragment.

TalkBack was picking up the text in the action bar, so if we hide it,
it will read the first body of text it encounters which would be "Scan
QR code. Pair device over Wi-Fi by scanning a QR code."

Bug: 152461605

Test: Manual. Enable TalkBack in Settings > Accessibility > TalkBack.
Navigate to Developer options > Wireless debugging > Pair by QR code.
When QR code scanner fragment opens, it should say "Scan QR code. Pair
device over Wi-Fi by scanning a QR code."

Change-Id: Ia91bf4ea215e6a377335f2b9dcf2acf10afd268e