Decouple VulkanManager from RenderThread

This CL allows for more than one VulkanManager to exist.
VulkanManager ctor are public allowing for classes other
than RenderThread to instantiate it.
Secondary VulkanManager can be used to render on a thread
other than RT.

Test: Ran HWUI unit tests and several apps
Change-Id: Ibfd76c86ff67e01617a500902bba7431b928f5c0
diff --git a/libs/hwui/Readback.cpp b/libs/hwui/Readback.cpp
index 76c5660..2ffda83 100644
--- a/libs/hwui/Readback.cpp
+++ b/libs/hwui/Readback.cpp
@@ -107,7 +107,7 @@
     if (Properties::getRenderPipelineType() == RenderPipelineType::SkiaGL) {
         mRenderThread.requireGlContext();
     } else {
-        mRenderThread.vulkanManager().initialize();
+        mRenderThread.requireVkContext();
     }
     if (!image.get()) {
         return CopyResult::UnknownError;