crypto changes for exports

* allow DecryptingPartInputStream to start reading from a specified offset in a file (to support headers, etc.)
* allow EncryptingPartOutputStream to write in appending mode to a file instead of overwriting
* "enhance" the getters and setters in IdentityKeyUtil and MasterSecretUtil to deal with getting MasterSecret objects from places other than SharedPreferences


How else could we get information about everything necessary to reconstruct the master secret?