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