Fix custom bootanimation

If the bootanimation is disabled and then you choose a custom bootanimation
the build.prop setting will not be updated and so the chosen bootanimation
will not be visible.

Change-Id: I0cea49a7b8e12fb431abddbe7852c481e4dfbbb5
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>