PackageParser: Use ExecutorService to parse certs and signatures

* Multithread the work of parsing package certificates and signatures
* Inspired by CAF commit: I57cb22b2d2e962975fe8f5e046b5bb4de67766d8
  "framework/base: use multithread to verify files contained in APK"

Change-Id: I8c4d3b09a9a6762bb321e1c84559ac1037a4e2de