FEATURE_LEANBACK implies that the device is a tv.

For managing the ui mode, consider the device to be a television if it
has either the FEATURE_TELEVISION or FEATURE_LEANBACK system features.

When leanback is available for non-television devices, we need to
reconsider the use of these features for setting the television ui mode.

Change-Id: I7b82372ce19067e010be9b42111816bab186f95b