Migrate legacy base::Callback type for ConnectionCallback

base::Bind and base::Callback are deprecated. Migrate the usages to
proper replacements.

The callback |ConnectionCallback| is called any time a new AVRCP device
is connected so it could be called multiple times and thus a
base::RepeatingCallback.

Bug: 272116782
Test: m .
Flag: EXEMPT mechanical refactor
Change-Id: I7b0bce14479458b9bfbcc8a4583c8ef16811f183