系统签名所需要的工具
signapk.jar -------------------->out/host/linux-x86/framework/signapk.jar (build/tools/signapk目录编译生成)
platform.pk8，platform.x509.pem ------------------> build/target/product/security
运用上述工具执行以下命令进行签名
java -jar signapk.jar platform.x509.pem platform.pk8 MDMDemo.apk signMDMDemo.apk
