libutils: introduce sp<T>::make

This is in preparation of doing what we did for SharedRefBase (hiding
operator new) so that clients can't accidentally construct
shared_ptr/unique_ptr or any other alternative memory management scheme
which would conflict with RefBase. You can see what ultimately happened
to SharedRefBase in frameworks/native CL
10d9ddf2e3da3ba3a425fb8396aaaec728e5fbdb.

The goal for this:
- promote use of 'sp<T>::make' over 'sp<T> .. = new T'
- make 'operator new' a private member of RefBase

Bug: 138956784
Test: libutils_test
Change-Id: I47f4d28edbf7534730c7b6fb1de748dd60f34e11