Hide speexdsp's echo cancellation implementation details behind an
EchoCanceller derived class. An AudioLayer may now instantiate
implementations of the EchoCanceller. This may be platform specific
compile-time or swapped out at runtime.

Introduces a null echo canceller for testing. As the speexdsp aec
is not currently functional, the null echo canceller is replaced
as the default implementation.

GitLab: #454
Change-Id: I169f1e9758afbed884fc42d9d78a69ce28d12fe2