Bug 997137: Inherit |DroidSocketImpl| from |UnixFdWatcher|, r=echou

This patch converts |DroidSocketImpl| to inherit from |UnixFdWatcher|.
|UnixFdWatcher| maintains a Unix file descriptor on the I/O loop, and
replaces similar functionality in |DroidSocketImpl|.

As a side effect, all file descriptors in |DroidSocketImpl| are now
non-blocking. This does change the way how the code works, but ensures
that it doesn't block on the I/O thread.