Avoid AllowedNetworkTypesListener NullPointerException.

If the CarrierServiceStateTracker's AllowedNetworkTypesListener
registers than radio power on, it gets the empty map. the map
gets null and then casues NullPointerException.
It is timeing issus, so add the protection on it.

Bug: 178443299
Test: atest CarrierServiceStateTrackerTest
Change-Id: Ia43f0117a2e1e573730554d878e9ce4cf9acad91
Merged-In: Ia43f0117a2e1e573730554d878e9ce4cf9acad91