SystemUI: Signal icon can not display after turn on/off airplane mode

When turn on/off airplane mode, sim state will change to absent
firstly, then change to ready. In the source code, there is a
judgement that if sim state is absent will set signal icon
invisible. However, when sim state change to ready, there is no
code to set signal icon visible again.

Add the code to set signal icon visible again.

CRs-Fixed: 648756

Change-Id: I5ef200de451418d12c446d4c9e516f953d30659b