staging: ion: make system and contig heaps modular

Add symmetric heap add/remove APIs and export symbols from
ion core that are needed to make both heaps modular.

Bug: 133508579
Test: ion-unit-test, rmmod, modprobe

Change-Id: Ic5f40936531936c9bfab48ea147108c019d28e2c
Signed-off-by: Sandeep Patil <sspatil@google.com>