AudioPolicyManager: always force AC3 on for TVs

Most TVs support AC3 but they do not report it in their EDID.
This changes forces support for AC3 unless turned off by Settings.
This change allows broadcast TV to work by default.

Bug: 24541671
Change-Id: I9e8cf194198db3811f91149014ef29e46972ad2c
Signed-off-by: Phil Burk <philburk@google.com>