ATV a11y shortcut (CL 1 of 3): Update key chord

Changed accessibility shortcut key chord from [HOME + DPAD_CENTER] to
[BACK + DPAD_DOWN]. (Followed logic in ag/1939791.)
Specified AudioAttributes of TV dialog ringtone to
USAGE_ASSISTANCE_ACCESSIBILITY since this dialog is meant to show up for
accessibility shortcut feature and that will also enable Android TVs to
play ringtone for the warning dialog.

Bug: 64346294
Test: Manually use key chord to trigger accessibility shortcut.
Change-Id: Iacb29400079551a3c3ded6f8391b7541bc1c9f29