display audio dialog when connecting low end dock

Display a dialog to enable the use of the dock audio connection
when a low end dock is connected for the first time.

Modify DockService to process docked and undocked messages even if
the device indicated is null (meaning the dock is not a bluetooth dock)
only for low end docks.

Bug 7302106.

Change-Id: I331d83a74fecf5f26b24bfc178342df414bd8153