custom bootanimation improvements

- custom boot animation picker disabled if boot animations are disabled,
    for consistency
- set custom boot animations to /data/local/bootanimation.zip so we
don't
    overwrite the system one.
- allow users to clear their custom boot animation

Change-Id: Ibdedf6c3c6bb55312a9088d5849af8f826cdffad
Signed-off-by: Roman Birg <romanbirg@gmail.com>