Fix systemui crash on boot

The process com.android.systemui would crash after phone booting with a
java.lang.NullPointerException when the signal status style is set to text

Change-Id: Ia7b58fa93b8591d0571b90a57a2fe0fe823b3f45
NFC polling mode (3/3)

Change-Id: Ia03413578d47e7cd0ffb3080778b6a1154cfc651