Enable default NAS for users who haven't done NAS migration

1. For users who haven't finished NAS migration (NAS_SETTINGS_UPDATED ==
false), we will enable the default NAS no matter user manually changed
the NAS setting or not.
2. Fix RuntimeException in PreferencesHelper when upgrading from Q- to T

Bug: 203689334
Bug: 208869735
Test: atest NotificationManagerServiceTest
Test: manually tested migration flow from SP1A, RQ3A, QQ3A with 1)factory
reset, 2) set NAS to none, 3) set NAS to different default NAS
Test: manually tested the bug affected flow: upgrade form RQ3A (factory
reset) to SP1A and then upgrade to this fix

Change-Id: Ic03c18fdf307beef49fe14031272df75c837cde2