Destroy prefetched layers when the context is destroyed

Also fix a few places preventing the tests from running
in SkiaVulkan, although that is still not enabled in general yet

Fixes: 283315634
Test: CanvasContext_buildLayerDoesntLeak unit test
Change-Id: I4d8a50db447e47afd8e8a267df332a173c88e888
diff --git a/libs/hwui/renderthread/CacheManager.h b/libs/hwui/renderthread/CacheManager.h
index 5e43ac2..bcfa4f3 100644
--- a/libs/hwui/renderthread/CacheManager.h
+++ b/libs/hwui/renderthread/CacheManager.h
@@ -64,12 +64,13 @@
     void unregisterCanvasContext(CanvasContext* context);
     void onContextStopped(CanvasContext* context);
 
+    bool areAllContextsStopped();
+
 private:
     friend class RenderThread;
 
     explicit CacheManager(RenderThread& thread);
     void setupCacheLimits();
-    bool areAllContextsStopped();
     void checkUiHidden();
     void scheduleDestroyContext();
     void cancelDestroyContext();