Add MultiDisplayList + memory leak fixes

MultiDisplayList can contain either a SkiaDisplayList
or a CanvasOpBuffer. However DisplayList itself
still points to the SkiaDisplayList-only wrapper
to avoid any std::variant or std::visit overhead
just yet.

Also fixes a memory leak in CanvasFrontend from an
uninitialized std::optional and a few minor leaks
in unit tests.

Test: build & hwui_unit passes

Fixes: 184680809
Change-Id: Ifa6b723b6456f5d3eeac1201e76f337250103d6f
Merged-In: Ifa6b723b6456f5d3eeac1201e76f337250103d6f
(cherry picked from commit d34d6cec97c8f1be92f676aeb79c83d57cf8c6ba)
diff --git a/libs/hwui/canvas/OpBuffer.h b/libs/hwui/canvas/OpBuffer.h
index 1237d69..8b5cdbb 100644
--- a/libs/hwui/canvas/OpBuffer.h
+++ b/libs/hwui/canvas/OpBuffer.h
@@ -64,7 +64,7 @@
     static constexpr auto STARTING_SIZE = PadAlign(sizeof(BufferHeader));
     using ItemHeader = OpBufferItemHeader<ItemTypes>;
 
-    OpBuffer() = default;
+    explicit OpBuffer() = default;
 
     // Prevent copying by default
     OpBuffer(const OpBuffer&) = delete;
@@ -135,7 +135,7 @@
 
     template <typename F>
     void for_each(F&& f) const {
-        for_each(std::forward<F>(f), ItemTypesSequence{});
+        do_for_each(std::forward<F>(f), ItemTypesSequence{});
     }
 
     void clear();
@@ -225,7 +225,7 @@
     }
 
     template <typename F, std::size_t... I>
-    void for_each(F&& f, std::index_sequence<I...>) const {
+    void do_for_each(F&& f, std::index_sequence<I...>) const {
         // Validate we're not empty
         if (isEmpty()) return;