ART: Make Handle trivially-copyable

Use default copy constructor and copy assignment to make Handle
trivially copyable. Do the same for MutableHandle.

Fix up unused Handle warnings. Add asserts in the HandleScope
test.

Bug: 32619234
Test: m test-art-host
Change-Id: I151f0bdbeeb131a6fc5c44610f345663ebe96c09
diff --git a/runtime/monitor_test.cc b/runtime/monitor_test.cc
index 4ee46dc..4fbfe47 100644
--- a/runtime/monitor_test.cc
+++ b/runtime/monitor_test.cc
@@ -401,14 +401,11 @@
   Thread* const self = Thread::Current();
   ThreadPool thread_pool("the pool", 2);
   ScopedObjectAccess soa(self);
-  StackHandleScope<3> hs(self);
+  StackHandleScope<1> hs(self);
   Handle<mirror::Object> obj1(
       hs.NewHandle<mirror::Object>(mirror::String::AllocFromModifiedUtf8(self, "hello, world!")));
-  Handle<mirror::Object> obj2(
-      hs.NewHandle<mirror::Object>(mirror::String::AllocFromModifiedUtf8(self, "hello, world!")));
   {
     ObjectLock<mirror::Object> lock1(self, obj1);
-    ObjectLock<mirror::Object> lock2(self, obj1);
     {
       ObjectTryLock<mirror::Object> trylock(self, obj1);
       EXPECT_TRUE(trylock.Acquired());