APK2 signer: verify min sdk version

[g-k]

It appears that we're signing everything with minSDK 18 regardless of key type. It'd be good to add unit or integration tests or both to verify the min SDK we sign with based on key type

autograph/signer/apk2/apk2.go

Lines 80 to 87 in 019e766

	case *ecdsa.PrivateKey:
	// ecdsa is only supported in sdk 18 and higher
	s.minSdkVersion = "18"
	log.Printf("apk2: setting min android sdk version to 18 as required to sign with ecdsa")
	default:
	log.Printf("apk2: setting min android sdk version to 9")
	s.minSdkVersion = "9"
	}

Possible changes:

• add a unit test with an ECDSA private key e.g. copy the apk_cert_with_ecdsa_sha256 config
• verify SDK versions for RSA and ECDSA Apk signers e.g. add apt -y install -t buster-backports aapt to the Dockerfile and check SDK versions aapt list -a | grep -i sdk