Add "operator bool" overload to android::sp.

This matches the overload on std::unique_ptr and friends.

Test: builds, boots
Bug: 13758960
Change-Id: Ieed9faa6b162c2a10fa7cf2b135c9b17564f6c88
diff --git a/libutils/include/utils/StrongPointer.h b/libutils/include/utils/StrongPointer.h
index 0c20607..ae6d9c8 100644
--- a/libutils/include/utils/StrongPointer.h
+++ b/libutils/include/utils/StrongPointer.h
@@ -82,9 +82,10 @@
 
     // Accessors
 
-    inline  T&      operator* () const  { return *m_ptr; }
-    inline  T*      operator-> () const { return m_ptr;  }
-    inline  T*      get() const         { return m_ptr; }
+    inline T&       operator* () const     { return *m_ptr; }
+    inline T*       operator-> () const    { return m_ptr;  }
+    inline T*       get() const            { return m_ptr; }
+    inline explicit operator bool () const { return m_ptr != nullptr; }
 
     // Operators