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/android/variable.go b/android/variable.go
index 3eee988..2057903 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -110,6 +110,7 @@
 	DateFromFile        *string `json:",omitempty"`
 
 	Platform_sdk_version              *int     `json:",omitempty"`
+	Platform_sdk_codename             *string  `json:",omitempty"`
 	Platform_sdk_final                *bool    `json:",omitempty"`
 	Platform_version_active_codenames []string `json:",omitempty"`
 	Platform_version_future_codenames []string `json:",omitempty"`