Fix aapt2 --min-sdk-version after finalized SDK

aapt2 --min-sdk-version was using AppsDefaultVersionName(), which
is OMR1 for a non-finalized SDK, but 8.1.0 after finalization.
Add PlatformSdkCodename() for non-finalized SDKs, use it for
DefaultAppTargetSdk(), and pass it for aapt2 --min-sdk-version.

Bug: 78224641
Test: TestAppSdkVersion in app_test.go
Change-Id: I622eaf92f8a940f79007c2a579536da325700b06
diff --git a/java/aar.go b/java/aar.go
index 57c752c..8b5b85e 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -134,7 +134,7 @@
 	// SDK version flags
 	switch sdkVersion {
 	case "", "current", "system_current", "test_current":
-		sdkVersion = proptools.NinjaEscape([]string{ctx.Config().AppsDefaultVersionName()})[0]
+		sdkVersion = proptools.NinjaEscape([]string{ctx.Config().DefaultAppTargetSdk()})[0]
 	}
 
 	linkFlags = append(linkFlags, "--min-sdk-version "+sdkVersion)