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


Don't love the idea of exposing these preferences directly.