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

libutils/include/utils/StrongPointer.h

@@ -72,6 +72,12 @@ public:
     template<typename U> sp(const sp<U>& other);  // NOLINT(implicit)
     template<typename U> sp(sp<U>&& other);  // NOLINT(implicit)
 
+    // Cast a strong pointer directly from one type to another. Constructors
+    // allow changing types, but only if they are pointer-compatible. This does
+    // a static_cast internally.
+    template <typename U>
+    static inline sp<T> cast(const sp<U>& other);
+
     ~sp();
 
     // Assignment
@@ -279,6 +285,12 @@ sp<T>::sp(sp<U>&& other)
     other.m_ptr = nullptr;
 }
 
+template <typename T>
+template <typename U>
+sp<T> sp<T>::cast(const sp<U>& other) {
+    return sp<T>::fromExisting(static_cast<T*>(other.get()));
+}
+
 template<typename T>
 sp<T>::~sp() {
     if (m_ptr)