Set the required Android API level when Vulkan validation layers are …

…automatically enabled for android_debug_unopt_arm64 builds (flutter#44116)

The Vulkan validation layer headers will not build if the Android API level is 23 or lower.

Currently the engine buildroot's default Android API level is 23.  The API level must be overridden for any configuration where Vulkan validation is enabled.

tools/gn

@@ -563,14 +563,16 @@ def to_gn_args(args):
   if args.fstack_protector:
     gn_args['use_fstack_protector'] = True
 
-  if args.enable_vulkan_validation_layers:
-    if args.target_os == 'android':
-      gn_args['android_api_level'] = 26
-    gn_args['enable_vulkan_validation_layers'] = True
+  enable_vulkan_validation = args.enable_vulkan_validation_layers
 
   # Enable Vulkan validation layer automatically on debug builds for arm64.
   if args.unoptimized and args.target_os == 'android' and args.android_cpu == 'arm64':
+    enable_vulkan_validation = True
+
+  if enable_vulkan_validation:
     gn_args['enable_vulkan_validation_layers'] = True
+    if args.target_os == 'android':
+      gn_args['android_api_level'] = 26
 
   # Enable pointer compression on 64-bit mobile targets. iOS is excluded due to
   # its inability to allocate address space without allocating memory.