Moved the place for the aec_debug_dump build flag and changed the name to apm_debug_dump

Currently, the aec_debug_dump buildflag can and is used to store data in the whole of
the audio processing module. Therefore a more appropriate name is apm_debug_dump which
also matches the names of the data dumping functionality. This CL makes that name change.

The CL also changes the WEBRTC_AEC_DEBUG_DUMP define to
WEBRTC_APM_DEBUG_DUMP == 1

Furthermore, this CL moves the buildflag to a more appropriate place.

BUG=webrtc:5298

Review-Url: https://codereview.webrtc.org/2300813004
Cr-Commit-Position: refs/heads/master@{#14026}