CallManager: add setAudioMode()

When in a call, different audio modes need to be applied based on phone type.
(For example, SIP call needs to be in MODE_NORMAL while GSM call in MODE_IN_CALL.)
Originally, it's handled in PhoneUtils.setAudioMode(). It makes more sense now
to handle the actual logic in CallManager.

Change-Id: I58d8f31d6b4afe22f88da442daac2010781de801