Hide heads up settings and show a notice if heads up is disabled.

This follows a UI pattern used at several other places, such as
daydream, screencast, profiles or blacklist settings.

Change-Id: I5635338ceccae8ba8450284d3078889333fb77c6
Signed-off-by: Anthony <Disciple3603@gmail.com>

Conflicts:
	src/com/android/settings/headsup/HeadsUpSettings.java