Wrap java.util.Timer in an interface so it can be mocked.

src/net/sf/briar/api/clock/SystemTimer.java

src/net/sf/briar/api/clock/Timer.java

src/net/sf/briar/api/crypto/MessageDigest.java

src/net/sf/briar/clock/ClockModule.java

src/net/sf/briar/db/DatabaseCleanerImpl.java

src/net/sf/briar/transport/KeyManagerImpl.java