Shard classloader classes lock

Used to guard adding and removing classes.

Previously we used the class linker classes lock, but this had
a deadlock issue since the reference processor may need to acquire
the lock to mark the classes of a class loader. Another thread could
be blocked trying to access weak globals while also holding the
class linker classes lock.

Bug: 27946564

(cherry picked from commit 1609e3a4)

Change-Id: Ic5cfe573c4e6822d49ad0862ffdd9d036e439a96