ion: check invalid values in ion_system_heap

ion_system_heap can only satisfy page alignment, and
ion_system_contig_heap can only satisify alignment to the
allocation size.  Neither can support faulting user mappings
because they use slab pages.

Change-Id: I895c2d4184c672f647f83a17aeb862985dc92f2c
Signed-off-by: Colin Cross <ccross@android.com>