libbinder_ndk: private SharedRefBase construction

SharedRefBase, like sp, has a weakref within it. This allows us to
promote to a strong ref when we only have a pointer to an object (for
instance when we're getting a binder from a binder transaction).
However, it means that the objects have an implicit ownership model.

So, this code is problematic:

     std::shared_ptr<IFoo> foo = std::make_shared<MyFoo>();
     ...
     // what other code will do when getting the binder from another
     // process, creating double ownership w/ shared_ptrs
     std::shared_ptr<IFoo> foo = foo->ref();

Here, we're hiding the use of the 'new' operator so that the initial
make_shared is impossible. If people always use 'SharedRefBase::make'
and 'SharedRefBase::ref' to get strong ownership of the object, then we
avoid this possibility.

Since we hide the 'new' operator, all heap allocation will be blocked.
So other possibilities of double-ownership (e.g. using std::shared_ptr)
will also fail.

One problem with this approach is that it still allows these objects to
be declared on the stack. This is an opportunity for improvement.

Bug: 149249948
Test: TH

Change-Id: I300008f1413474c9e78dd57217f57338a3528db0