Remove read barriers in InternTable::Table::AddInternStrings

Read barriers are not safe to do on spaces that are not yet added
to the heap since they will be detected as heap corruption. These
read barriers are also not necessary in this case since strings
objects are immutable.

Test: test-art-host
Bug: 117803941
Change-Id: I46b64ae894611fe15ee1374fe71200471bd7d61a