ion: fix overflow and list bugs in system heap

Fix a few bugs in ion_system_heap:

Initialize the list node in the info block.

Don't store size_remaining in a signed long, allocating >2GB
could overflow, resulting in a call to sg_alloc_table with
nents=0 which panics.  alloc_largest_available will never
return a block larger than size_remanining, so it can never
go negative.

Limit a single allocation to half of all memory.  Prevents a
large allocation from taking down the whole system.

Change-Id: I7fcbd7e1d5b4d482d7612d80b6c9e8e24466f1d8
Signed-off-by: Colin Cross <ccross@android.com>