[vulkan] Add feature flag

bug: 111137294

This allows us to build the HAL, but not activate it unless a
feature flag is enabled.

If the feature flag is not enabled, the stub hal is used.

Change-Id: I678a1eea0f36fd1574c78a400676f7189a8fc53c
diff --git a/system/vulkan_enc/ResourceTracker.h b/system/vulkan_enc/ResourceTracker.h
index cdbb54d..c63dfc0 100644
--- a/system/vulkan_enc/ResourceTracker.h
+++ b/system/vulkan_enc/ResourceTracker.h
@@ -143,6 +143,7 @@
     VkDeviceSize getNonCoherentExtendedSize(VkDevice device, VkDeviceSize basicSize) const;
     bool isValidMemoryRange(const VkMappedMemoryRange& range) const;
     void setupFeatures(const EmulatorFeatureInfo* features);
+    bool hostSupportsVulkan() const;
     bool usingDirectMapping() const;
     void deviceMemoryTransform_tohost(
         VkDeviceMemory* memory, uint32_t memoryCount,
@@ -167,4 +168,4 @@
     std::unique_ptr<Impl> mImpl;
 };
 
-} // namespace goldfish_vk
\ No newline at end of file
+} // namespace goldfish_vk