Only disable volume UI for TV playback devices in full volume

Volume UI flags for full volume devices are disabled in
`#sendVolumeUpdate(..)`.

TV Playback devices in full volume should not show volume UI. Even if HDMI
CEC volume control is disabled, the TV volume might be controlled via
other mechanisms.

Variable volume devices should always show volume UI, no matter the HDMI
CEC volume control state.

Bug: 161174563
Test: manual
Change-Id: Iab53e7db202428723429ff53b2314f011230b989