Briar crashed when run in a device without bluetooth or without camera
such as an emulator.

Closes #514

When more than one invitation socket is opened, Alice should pick which one to use and Bob should use whichever one Alice picks. This fixes a race condition where each party picked a different socket and closed the other.

Same exponential backoff approach as Bluetooth.

The polling interval increases exponentially each time polling is unsuccessful, up to a maximum of 60 minutes. The interval is reset to 2 minutes whenever a connection is made and whenever Bluetooth is re-enabled.

Removed methods that are no longer needed for Java 6, moved the remaining method into the utils directory.

"synchLock" will become confusing when we have lots of objects with "sync" in the name.

- replaced use of Object instance mutex with a private final Lock object
- replaced Object signaling with specific condition signalling

DuplexOutgoingSession flushes its output stream if it's idle for a
transport-defined interval, causing an empty frame to be sent. The TCP
and Tor plugins use a socket timeout equal to twice the idle interval to
detect dead connections.

See bugs #27, #46 and #60.