SystemUI: transparency control overhaul

Allow more customization. TransparencyManager's responsibility
is to apply the values it is being fed from Settings.System set by
ROMControl.

Tablets do not use transparency currently, but it shouldn't throw
any errors.

Change-Id: I11c1578938e337285d99e9fa1fbbd537a4ee43ec
Signed-off-by: Roman Birg <romanbirg@gmail.com>