[Audiosharing] Update dialog btn when start sharing with no extra connected device

There are two btns now: Pair new device and Show QR code
Pair new device -> go to Pair new device page
Show QR code -> go to QR code page

Test: atest
Flag: com.android.settingslib.flags.enable_le_audio_sharing
Bug: 305620450

Change-Id: Id5ddb66f1bbc3b02228a0d639bf30cdc40bef265