Invalidate Skia shader cache if Vulkan driver has changed

Test: Ran apps several time and observed shader cache validation
Change-Id: I7c2620b9a0eefcb7acedc86647ecf44805518b2f
diff --git a/libs/hwui/renderthread/VulkanManager.h b/libs/hwui/renderthread/VulkanManager.h
index c2d1802..54333f3 100644
--- a/libs/hwui/renderthread/VulkanManager.h
+++ b/libs/hwui/renderthread/VulkanManager.h
@@ -82,6 +82,8 @@
 
     sk_sp<GrContext> createContext(const GrContextOptions& options);
 
+    uint32_t getDriverVersion() const { return mDriverVersion; }
+
 private:
     friend class VulkanSurface;
     // Sets up the VkInstance and VkDevice objects. Also fills out the passed in
@@ -178,6 +180,7 @@
     };
     SwapBehavior mSwapBehavior = SwapBehavior::Discard;
     GrVkExtensions mExtensions;
+    uint32_t mDriverVersion = 0;
 };
 
 } /* namespace renderthread */