HDMICEC: handle hdmi_control help command

The onHelp function is supposed to be invoked (by handleDefaultCommands)
for the "help" command (in addition to the null command).

Bug: 338392163
Test: adb shell cmd hdmi_control help
(cherry picked from https://android-review.googlesource.com/q/commit:a233e5531653cc5a4e6dbb563d2daefcd1988a14)
Merged-In: I1a6743ba793493322e9ccafff2861d4418ba4d7c
Change-Id: I1a6743ba793493322e9ccafff2861d4418ba4d7c