The docs say this can happen for a directory if there's an I/O error.

This allows for swiping left/right to read other posts by using
a ViewPager.

This hasn't been done as a separate activity, but with
fragments, so both can share the `BlogPersistentData` without
needing to reload it.

Closes #428

Will currently fail at runtime; requires a public key and a server onion.

Modified the project structure, removed module extension and went instead for a non-complete core dependency graph

Semi-encapsulated the core/api dependency graphs and created a proper structure to load eager singletons

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.

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

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.

Android doesn't currently store bundles persistently, so it's premature
to protect against accidental information leaks through persistent
bundle storage. Protecting against deliberate information leaks by the
OS is probably futile, so there's currently no need for bundle
encryption.

Only the ARM binary is included at present. Better control of the Tor
process is needed - it's possible for it to be left running when Briar
exits, and if the cookie file is deleted it's no longer possible to stop
the process.

The UI may access the DB in response to UI or DB events; to maintain a
consistent view of the DB's contents, the tasks performing these
accesses must be prevented from overlapping, and must produce consistent
results if reordered. A single-threaded executor and latches are used to
prevent tasks from overlapping, without blocking non-UI access to the
DB.