Remove SystemSettingSwitchPreference.

We should not introduce one-off UI patterns when there are plenty of
examples where the same usage pattern (a feature on its own
PreferenceScreen with a global enable switch in the action bar) is
handled differently everywhere else (e.g. daydream, cast display,
blacklist). This makes for inconsistent UX.
Also, there are discoverability issues: As there's no other case of a
preference with an embedded control widget opening another preference
screen, users won't expect to find another screen behind it.
Additionally - but entirely subjective - the control looks plain ugly,
as the preference padding is too large for the large switch.

Change-Id: I61284e961babefa3748102b05055fb1d03b079b0
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
Settings: Add heads up quicksettings tile. (2/2)

Change-Id: Ib1db69671d9f5c2c6dfa019739674fee50df9340
[2/2] Profile: Remove screen lock mode
[2/2] Profile: Screen timeout
[2/2] Battery light: Add 100% charged level
[2/2] Buttons: Switch to last app
[2/2] Global actions: Mobile data
[2/2] Global actions: Wifi
[2/2] Quick pulldown: center
[2/2] Smart pulldown
[2/2] Volume panel timeout
[2/2] Low battery warning policy
[2/2] Recent apps: Clear all button: Location
[2/2] Recent apps: RAM bar
[2/2] Network traffic indicator
[2/2] Floating window
[2/2] Lockscreen notifications
[2/2] Hover