Removed Commons IO, which we were only using as a replacement for
File.getFreeSpace() on desktop plaftorms.

Note: The Huawei U8210 (Android 2.1) doesn't have all the Java 1.6
standard library methods, and crashes if they're called. Specifically,
String.isEmpty() and NetworkInterface.supportsMulticast() are missing,
so the changes removing those methods were not reverted.

The method used by Commons IO to get the available disk space fails on
Android devices that lack a df binary - use the Android API instead.

This removes the other projects' dependency on the Android API.

See issue #3612607.

Both sides now close the connection after exchanging confirmation
results unless both results are positive.

Some Android devices require a lock to be held while using multicast, to
disable the packet filter that normally filters out multicast packets.

Fixes issue #3611924.

Partially addresses issue #3611924.

Fixes issue #3612299.

Fixes issue #3612607.

CryptoExecutor and DatabaseExecutor now use bounded thread pools with
unbounded queues, since running too many tasks in parallel is likely to
harm performance; IncomingConnectionExecutor, PluginExecutor and
ReliabilityExecutor use unbounded thread pools with direct handoff,
since their tasks may run indefinitely. There are no longer any bounded
executors, and all executors discard tasks when shutting down, which
fixes issue #3612189.

Responsibility for starting and stopping services has been moved from
BriarService in briar-android to LifecycleManagerImpl in briar-core.
However, BriarService is still responsible for stopping the
Android-specific executors, which is ugly. It would be better if
executors registered themselves with LifecycleManager.

Fixes issue #3611923.