releasetools: Add method to replace cert fingerprint

* Some of the apps we bundle rely on a list of allowed
  packagename to fingerprint mapping stored in a xml
  file on system
* Replace cert fingerprint with the new key
* Should be safe to just loop this on all keys, replacing
  as needed.

Change-Id: Iaac5e41305d831db8aa2f1cd2f55aab8b48be4a6