Make interned strings non-movable.

Make interned strings non-movable.

At present objects referenced from dex files must have stable reference
values.  With this change, only non-moving strings are interned.  If a
user interns a movable string a non-moving copy is made and the copy is
added to the intern table.

As part of this change, the internal string hash code access routine will
update the hash code slot of a string object.  In addition, StringObject
has been made a subclass of Object eliminating various down-casts that
would otherwise be explicitly required.

Change-Id: I6b015b972aac44948470c0034ad17e5eef456aeb