CEC: Revisit the behavior of HDMI_SYSTEM_AUDIO_ENABLED

Currently the system setting, HDMI_SYSTEM_AUDIO_ENABLED, is used to
store the latest system audio mode status so that TV can keep this
status over reboot. But because the name is a little confusing and the
behavior isn't intuitive, it is likely to use this in a wrong way.
This change renames this setting to HDMI_SYSTEM_AUDIO_CONTROL_ENABLED
and tweak the purpose of it. Now, it will act more like a switch for
System Audio Control feature, so user can disable or enable this feature
entirely. With this way, implementation of audio output option will
also become easier.

Bug: 31449672
Test: Tested on archer
Change-Id: Ice8717135272d4b86665a3452bfe7527c0d6c08b
(cherry picked from commit 7b7aa8fb31ccf0cd3f36162a52f080263dd89e77)