ion: Improve support for heap walking

Clients may wish to implement custom functions on a particular
heap ID. That function assumes that the heap ID has a specific heap
type. Make that requirement explicit by only calling the custom
function if both the ID and type match.

Change-Id: Ie746362a19a22dceb6e47148d67901d483778a85
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>