ion: Ensure correct heap type set during heap drain

Since process_one_shrink is attempting to make a secure allocation
correctly set the heap type to system secure heap and not system heap
type.

This is now required since we validate the buffer heap type when the
allocation is processed.

Without this change the allocation will fail as it is attempting to do a
secure allocation from the system heap.

Change-Id: I54fbc2bc6477b47f8e4e92e797b27840d42dde79
Signed-off-by: Liam Mark <lmark@codeaurora.org>