Update comment for TV version of inattentive_sleep_warning_message

Clarify that this is for a TV device, and that the expected
interaction is a TV remote button press.
Helps guide translators to find an appropriate translation.

Bug: 173363429
Test: `m SystemUI` still compiles, phew
Change-Id: I0c1ebad503a3c68ce78f64de1286d089aeebf324