Provide writer preference option in rwlock.

Previous implementation of rwlock contains four atomic variables, which
is hard to maintain and change. So I make following changes in this CL:

1. Add pending flags in rwlock.state, so we don't need to synchronize
between different atomic variables. Using compare_and_swap operations
on rwlock.state is enough for all state change.

2. Add pending_lock to protect readers/writers waiting and wake up
operations. As waiting/wakeup is not performance critical, using a
lock is easier to maintain.

3. Add writer preference option.

4. Add unit tests for rwlock.

Bug: 19109156

Change-Id: Idcaa58d695ea401d64445610b465ac5cff23ec7c