Change /dev/ion from read-only to read-write

Even though /dev/ion can allocate memory when opened in read-only mode,
some processes seem to unnecessarily open it in read-write mode.
This doesn't seem to be harmful, and was originally allowed in
domain_deprecated. Re-allow it.

Bug: 25965160
Change-Id: Icaf948be89a8f2805e9b6a22633fa05b69988e4f