libutils: add sp::cast method

Previously, sp::sp(T*) internally had a static cast, and people
frequently wrote code like this:

    sp<A> a = ...;
    sp<B> b(a.get()); // implicit static cast

Luckily, none of the other sp constructors have this implicit cast. So,
for explicit code, rather than making those use static_cast internally,
adding an sp::cast function.

Bug: 184190315
Test: use in libbinder
Change-Id: Id205c88d03e16cf85ccb8f493ce88b4bbc65a688