PgpKeyOperation: factor out all android dependencies

Conflicts:
	OpenPGP-Keychain/src/main/java/org/sufficientlysecure/keychain/pgp/PgpKeyOperation.java
	OpenPGP-Keychain/src/main/java/org/sufficientlysecure/keychain/service/KeychainIntentService.java
test: add junit+robolectric to buildfiles and stub test file
Merge pull request #501 from Valodim/master

factor out android dependencies from PgpKeyOperation, and add JUnit/Robolectric testing stubs