Removed double-encryption of shared secrets.

api/net/sf/briar/api/crypto/SecretStorageKey.java

components/net/sf/briar/crypto/CryptoComponentImpl.java

components/net/sf/briar/crypto/CryptoModule.java

components/net/sf/briar/crypto/SharedSecret.java

test/net/sf/briar/ProtocolIntegrationTest.java

test/net/sf/briar/crypto/CryptoComponentTest.java

test/net/sf/briar/crypto/SharedSecretTest.java

...f/briar/transport/batch/BatchConnectionReadWriteTest.java