[Audiosharing] Impl the join audio sharing dialog.

Show the dialog when the first or second eligible remote device connected during the
sharing session;
Or when the second eligible remote device connected and
no sharing session.

Flagged with enable_le_audio_sharing

Bug: 305620450
Test: Manual
Change-Id: If59025d60821b00c169e0d739f4b68c31ce4bbbd