ion: Improve ION allocation paths

Clean up some of the ION heap interfaces so that they are
more inline/uniform with respect to each other.

Change-Id: I4edabc2c8ccb533898540ceda1fd6aacc2e2e56a
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>