Explicitly specify min API 31 for tzdata module

Now that API 31 is finalized, the Android.bp can be updated to reflect
the fact that the tzdata module is API release-specific.

Bug: 174024269
Test: build only
Change-Id: I466d94e1a8e4005fb753863940f16a4c2510ae61
diff --git a/apex/Android.bp b/apex/Android.bp
index 9521f55..6a92104 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -24,7 +24,7 @@
 apex_defaults {
     name: "com.android.tzdata-defaults",
     updatable: true,
-    min_sdk_version: "30",
+    min_sdk_version: "31",
 
     // Use a custom AndroidManifest.xml used for API targeting.
     androidManifest: ":com.android.tzdata-androidManifest",