AudioService: log focus changes and limit focus stack size

Log audio focus request and abandon commands.
Limit the size of the audio focus stack to prevent malicious
  applications trying to run the process out of memory.

Test: use media apps and speech reco, check dumpsys audio
Bug: 67055749
Change-Id: I529529fe40f33a8f67c64c3c61ba22d9530fb4d9