CEC: Remove the TODO in HdmiPlaybackClient class

HdmiControlService.oneTouchPlay method is already switching the
long running one touch play process to the service thread from
the UI thread and is also able to handle multiple OTP requests
in parallel. Remove TODO to use PendingResult.

Bug: 159901363
Test: None
Change-Id: Ife8c8268edb8a40255a52dd1eda2ab60020803c8