ion: Restore GKI system heap implementation

Restore the generic system heap implementation such that it does not have
any MSM specific additions as of android11-5.4
commit a9a13eee ("Merge 5.4.45 into android-5.4-stable").

Change-Id: Iedc33377d5fa64513d43459d305ff98ce2091a61
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>