Don't add APEX keysets to KeySetManagerService

There are orphaned keysets during boot because APEX package
settings are not persisted.

This will also improve boot time as we don't create keysets
for APEX.

Bug: 225756739
Test: presubmit
Change-Id: I7b781b26b45d0e08be9a1e8750dbee2b2c5fdb6f