RefBase: update sp and wp to allow constinit

A constexpr ctor is useful for static initialization.
We also modernize the unit test for static variable best practices.

Test: atest libutils_binder_test
Change-Id: If42c0939fb1a2dcb8eb101ed0e10051f67e2150d

libutils/binder/RefBase_test.cpp

@@ -300,8 +300,8 @@ private:
     std::atomic<int>* mDeleteCount;
 };
 
-static sp<Bar> buffer;
-static std::atomic<bool> bufferFull(false);
+[[clang::no_destroy]] static constinit sp<Bar> buffer;
+static constinit std::atomic<bool> bufferFull(false);
 
 // Wait until bufferFull has value val.
 static inline void waitFor(bool val) {
@@ -380,8 +380,8 @@ TEST(RefBase, RacingDestructors) {
     }  // Otherwise this is slow and probably pointless on a uniprocessor.
 }
 
-static wp<Bar> wpBuffer;
-static std::atomic<bool> wpBufferFull(false);
+[[clang::no_destroy]] static constinit wp<Bar> wpBuffer;
+static constinit std::atomic<bool> wpBufferFull(false);
 
 // Wait until wpBufferFull has value val.
 static inline void wpWaitFor(bool val) {

libutils/binder/include/utils/RefBase.h

@@ -404,7 +404,7 @@ class wp
 public:
     typedef typename RefBase::weakref_type weakref_type;
 
-    inline wp() : m_ptr(nullptr), m_refs(nullptr) { }
+    inline constexpr wp() : m_ptr(nullptr), m_refs(nullptr) { }
 
     // if nullptr, returns nullptr
     //

libutils/binder/include/utils/StrongPointer.h

@@ -30,7 +30,7 @@ template<typename T> class wp;
 template<typename T>
 class sp {
 public:
-    inline sp() : m_ptr(nullptr) { }
+    inline constexpr sp() : m_ptr(nullptr) { }
 
     // The old way of using sp<> was like this. This is bad because it relies
     // on implicit conversion to sp<>, which we would like to remove (if an