systemui: SignalClusterTextView fixes and optimizations

- Optimize the SignalClusterTextView layout, but removing an unnecessary LinearLayout hierarchy
- Optimize the SignalClusterTextView view by hidding the entire view instead the cluster view
- Set start visibility of SignalClusterTextView to gone and update its state in onAttachToWindow
event. This should prevents bug CYAN-4410.
- Prevents NPE in MSIM devices when TEXT style mode is configured for the signal cluster style

Change-Id: Ib91fde693d5013bf058f161cc175923913fdfb7d
JIRA: CYAN-4410
Issue: https://jira.cyanogenmod.org/browse/CYAN-4410
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>