Read global ASAN_OPTIONS from a file.

Instead of setting global ASAN_OPTIONS in immutable init.environ.rc,
load them from a file that can be changed later. The file has to be
on the /system partition to both be editable and available at the
early stages of boot.

Also add allocator_may_return_null=1 as that is closer to the
non-ASan allocator behavior.

Bug: 22846541
Change-Id: Ib0f41393c528f2e7cb398470e41f50abf5f4f455