Support pw::Allocator in RefBase and sp

This will allow for custom allocators to be used by
types implementing RefBase and using sp when the
ANDROID_UTILS_USE_PW_ALLOCATOR flag is specified.

If the allocator is not set, we will use standard
new/delete as we do not control where the memory is
allocated. A user may not use sp to allocate the
memory, so we must use default new/delete. If the
user sets the global allocator, they are expected
to use it to allocate all objects if not using sp.

We explicitly disallow allocating arrays of types
using these constructs. Instead, we provide a simple
buffer allocation macro. This reduces the number of
combinations we need to support.

Bug: 416545855
Flag: EXEMPT bugfix
Test: atest libutils_binder_test
Change-Id: I1d1a81dc345d88c94b2ca70c0b04fc32d9f04647