ANDROID: binder: Don't BUG_ON(!spin_is_locked()).

Because is_spin_locked() always returns false on UP
systems.

Use assert_spin_locked() instead, and remove the
WARN_ON() instances, since those were easy to verify.

Bug: 64073116
Change-Id: I9080991c6d67e91928282a3ee64db23e50c7d66a
Signed-off-by: Martijn Coenen <maco@android.com>