Fix <System Audio Mode Request> handler

CUrrent implemenetation does not use the parameter to route to ARC when
TV sends out Request System Audio Mode on. Instead it request Active
Source when initializing ARC.

Since ARC and System Audio Mode are independent, we should rely on
the physical address sent by <System Audio Mode Request> to locate the
current Active Source.

Also we don't need to terminate ARC when System
Audio Mode is turned off.

Test: manual
Bug: 119151888
Change-Id: I18b0b5225fe57fafde78db15663d9f0a9e225e82
(cherry picked from commit 679dd60e6dbc00141641637af3c18e581db7197b)