Themes: Avoid NPE when setting text in TickerView

It is possible, with the right timing, to receive an NPE in TextSwitcher
when a notification comes in during recreation of the status bar because
of a theme change.

Change-Id: Ia35ad0346f232df905ecff69158b71f3bf61a163
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0