Ringbuffer stops logging after wifi toggle

Forces ringbuffer updates before writing zipped files to bugreport.
Also properly resets the ringbuffer callback when it's deregistered.

Bug: 161307741
Test: make vts -j64 && vts-tradefed run commandAndExit vts --module VtsHalWifiV1_0Target
Test: manual verification with triggering a bugreport
Test: manual verification with wifi toggles
Change-Id: I74deffda917054e20cfcf4f0dd90d778626c89d3