CommandProcessor: add a mock

Add the MockCommandProcessor class, so that we can
test how upper layers of wifilogd interact with the
CommandProcessor.

Along the way:
- make CommandProcessor::ProcessCommand() a virtual
  method
- make the CommandProcessor dtor virtual

While there: fix some typos in TODOs.

Bug: 32779626
Test: ./runtests.sh (on angler)
Change-Id: I12d9560ed3a1bcd6bc90ffe444dc3ac6a473328e
5 files changed