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


This weird. At minimum it seems like the parameter should be IdentityKeyPair, but it also don't really feel like this belongs here.