Keyguard: Add null checks for show spn and plmn flags

The showSpn and showPlmn hash tables could have null
value for subs whose spn, plmn information has not been
received. Accessing such values will lead to a null
pointer exception.

Add appropriate null check before accessing flags that
control display of spn/plmn strings.

Change-Id: I552d02d8ddd1515c2240bcb8dea62ea860030682
CRs-Fixed: 784253