libion: make tests work with Ion from common kernel.

The tests do not care about legacy ION interface and are intended to
concentrate for kernels 4.14+ (new API). Running these tests
successfully during changes in ION will keep us honest with the upcoming
heap-as-module changes in ION.

Test: ./ion-unit-tests on cuttlefish.
Bug: 133508579

Change-Id: I2a31d09d09deb5be9ae84c4981cdf4b3c2cceb4c
Signed-off-by: Sandeep Patil <sspatil@google.com>