Hook up system audio mode

Whenever system audio mode is changed, it should notify to AudioManager
so that AudioManager changes internal output or mute status accordingly.
To have more clear cut between last setting and current system audio mode,
renamed mSystemAudioMode into mSystemAudioActivated and added
getLastSystemAudioMode() which returns last system audio mode value stored
in global settings (NVM).

Bug: 16002150
Change-Id: I5d01d879c1c5a9a3f4b0cf2d25e9e74c26c52fa0