Enable/disable system audio mode by changing the setting value

Detect a change of a setting value, Global#HDMI_SYSTEM_AUDIO_ENABLED, in
order to let the system audio mode enabled or disabled according to the
changed value. It allows to enable/disable the system audio mode using
adb.

Bug: 26981641
Change-Id: Iea320c44a3ea7a5aa4d5eb89f001db27312f1223
(cherry picked from commit 1e5703a6a5441c2a118a4635ec7cb6c26496c5b8)