aec: move aec implementations into EchoCanceller

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