Updates: Use appropriate certificate to verify and purchase apps

* Only show updates for devices that has atleast one matching certificate
* On Android 9.0+ devices, purchase apps with latest certificate's hash while updating
  to support key rotations

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>

app/src/main/java/com/aurora/extensions/Signature.kt

@@ -0,0 +1,12 @@
+package com.aurora.extensions
+
+import android.content.pm.Signature
+import java.security.cert.CertificateFactory
+import java.security.cert.X509Certificate
+
+fun Signature.generateX509Certificate(): X509Certificate {
+    val certificateFactory = CertificateFactory.getInstance("X509")
+    return certificateFactory.generateCertificate(
+        this.toByteArray().inputStream()
+    ) as X509Certificate
+}