Always make feature available, even when no SIMS are currently installed

If we only enable the feature when SIMs are installed at startup, there
is a bug where inserting the first SIM after the service starts will
still allow 2G networking.

Bug: 401244615
Test: atest DisallowCellular2GTest
Flag: android.security.aapm_feature_disable_cellular_2g
Change-Id: Ic98b8fa04efc639ba9504014eb79406c7a8273d8