Add a ZeroingAllocator::rebind<Other> for Other==char

Newer versions of libc++ check that an allocator can be rebound to the
same element type. We need to add a rebind member to ZeroingAllocator
to fix this compiler error:

prebuilts/clang/host/linux-x86/clang-r498229/include/c++/v1/vector:376:19: error: static assertion failed due to requirement 'is_same<android::vold::ZeroingAllocator, std::allocator<char>>::value': [allocator.requirements] states that rebinding an allocator to the same type should result in the original allocator
    static_assert(is_same<allocator_type, __rebind_alloc<__alloc_traits, value_type> >::value,
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It likely doesn't matter in practice because this allocator is only
used with std::vector, which probably doesn't use allocator rebinding,
because it won't allocate an internal node type (e.g. unlike std::map,
std::list, etc).

Alternatively, ZeroingAllocator could be changed to a
ZeroingAllocator<T> that can zero arbitrary types, but it doesn't seem
necessary currently, and types other than char wouldn't be used.

Bug: b/175635923
Test: treehugger
Change-Id: I42e9d8f02a18637fc67e94cc1358d2ed733a7268