Fields in Android UI objects that are accessed from background threads
must be declared volatile. UI objects use data attached to DB events to
avoid DB lookups, which complicates the UI code but should improve
performance.

Generate a random UUID instead of using a fixed UUID. Close sockets when
exceptions are thrown (not doing so can cause problems with subsequent
sockets on Android). Use a semaphore with tryAcquire() instead of a lock
when making alien calls, to avoid possible deadlocks.

Rejecting codes greater than 2^19 - 1 creates a confusing user
experience.