soong: Add `aapt_version_code` default

This appends `--version_code=$(date -u +%Y%m%d)` to aapt flags, which is
useful for flushing some caches upon system updates.

Change-Id: I6575b878f09c1c3138e12abc34d39405f51245e7
diff --git a/config/Android.bp b/config/Android.bp
index 73cd727..218d2b0 100644
--- a/config/Android.bp
+++ b/config/Android.bp
@@ -1,5 +1,25 @@
 // Omni soong configs
 soong_config_module_type {
+    name: "aapt_version_code",
+    module_type: "java_defaults",
+    config_namespace: "omniGlobalVars",
+    value_variables: ["aapt_version_code"],
+    properties: ["aaptflags"],
+}
+
+aapt_version_code {
+    name: "aapt_version_code_defaults",
+    soong_config_variables: {
+        aapt_version_code: {
+            aaptflags: [
+                "--version-code",
+                "%s",
+            ],
+        },
+    },
+}
+
+soong_config_module_type {
     name: "vendor_init",
     module_type: "cc_defaults",
     config_namespace: "omniGlobalVars",