Prevent unnecessary building APN candidates

When the APN context is already in disconnected state, there
is no need to build the waiting APN list, which can potentially
cause preferred APN gets deleted.

Bug: 144400654
Test: Manual
Merged-In: I8dd8edc6a9485148dde7b9b6b4326baed1632ea4
Change-Id: I8dd8edc6a9485148dde7b9b6b4326baed1632ea4
(cherry picked from commit 9a00218d)